Applying The Anova Test Education Essay
Chapter 6
ANOVA
When you want to compare means of more than two groups or levels of an independent variable, one way ANOVA can be used. Anova is used for finding significant relations. Anova is used to find significant relation between various variables. The procedure of ANOVA involves the derivation of two different estimates of population variance from the data. Then statistic is calculated from the ratio of these two estimates. One of these estimates (between group variance) is the measure of the effect of independent variable combined with error variance. The other estimate (within group variance) is of error variance itself. The F-ratio is the ratio of between groups and within groups variance. In case, the null hypothesis is rejected, i.e., when significant different lies, post adhoc analysis or other tests need to be performed to see the results.
The Anova test is a parametric test which assumes:
Population normality – data is numerical data representing samples from normally distributed populations
Homogeneity of variance – the variances of the groups are “similar”
the sizes of the groups are “similar”
the groups should be independent
ANOVA tests the null hypothesis that the means of all the groups being compared are equal, and produces a statistic called F. If the means of all the groups tested by ANOVA are equal, fine. But if the result tells us to reject the null hypothesis, we perform Brown-Forsythe and Welch test options in SPSS.
Assumption of Anova: Homogeneity of Variance. As such homogeneity of variance tests are performed. If this assumption is broken then Brown-Forsythe test option and Welch test option display alternate versions of F-statistic.
Homogeneity of Variance: If significance value is less than 0.05, variances of groups are significantly different.
Brown-Forsythe and Welch test option: If significance value is less than 0.05, reject null hypothesis.
Anova: If significance value is less than 0.05, reject null hypothesis.
Post Hoc analysis involves hunting through data for some significance. This testing carries risks of type I errors. Post hoc tests are designed to protect against type I errors, given that all the possible comparisons are going to be made. These tests are stricter than planned comparisons and it is difficult to obtain significance. There are many post hoc tests. More the options, stricter will be the determination of significance. Some post hoc tests are:
Scheffe test- allows every possible comparison to be made but is tough on rejecting the null hypothesis.
Tukey test / honestly significant difference (HSD) test- lenient but the types of comparison that can be made are restricted. This chapter will show Tukey test also.
One way ANOVA
Working Example 1 : One-way between groups ANOVA with post-hoc comparisons
Vijender Gupta wants to compare the scores of CBSE students from four metro cities of India i.e. Delhi, Kolkata, Mumbai, Chennai. He obtained 20 participant scores based on random sampling from each of the four metro cities, collecting 100 responses. Also note that, this is independent design, since the respondents are from different cities. He made following hypothesis:
Null Hypothesis : There is no significant difference in scores from different metro cities of India
Alternate Hypothesis : There is significant difference in scores from different metro cities of India
Make the variable view of data table as shown in the figure below.
Enter the values of city as 1-Delhi, 2-Kolkata, 3-Mumbai, 4-Chennai.
Fill the data view with following data.
City Score
1 400.00
1 450.00
1 499.00
1 480.00
1 495.00
1 300.00
1 350.00
1 356.00
1 269.00
1 298.00
1 299.00
1 599.00
1 466.00
1 591.00
1 502.00
1 598.00
1 548.00
1 459.00
1 489.00
1 499.00
2 389.00
2 398.00
2 399.00
2 599.00
2 598.00
2 457.00
2 498.00
2 400.00
2 300.00
2 369.00
2 368.00
2 348.00
2 499.00
2 475.00
2 489.00
2 498.00
2 399.00
2 398.00
2 378.00
2 498.00
3 488.00
3 469.00
3 425.00
3 450.00
3 399.00
3 385.00
3 358.00
3 299.00
3 298.00
3 389.00
3 398.00
3 349.00
3 358.00
3 498.00
3 452.00
3 411.00
3 398.00
3 379.00
3 295.00
3 250.00
4 450.00
4 400.00
4 450.00
4 428.00
4 398.00
4 359.00
4 360.00
4 302.00
4 310.00
4 295.00
4 259.00
4 301.00
4 322.00
4 365.00
4 389.00
4 378.00
4 345.00
4 498.00
4 489.00
4 456.00
Click on Analyze menuƒ Compare Meansƒ One-Way ANOVA….One-Way ANOVA dialogue box will be opened.
Select Student Score(dependent variable) in Dependent List box and City(independent variable) in the Factor as shown in the figure below.
Click Contrasts… push button. Contrasts sub dialogue box will be opened. See that all the settings remain as shown in the figure below. Click Continue to close this sub dialogue box and come back to One-Way ANOVA dialogue box.
Click Post Hoc… push button. Post Hoc sub dialogue box will be opened. See that all the settings remain as shown in the figure below. Click Tukey test and Click Continue to close this sub dialogue box and come back to One-Way ANOVA dialogue box. Also note that significant level in this sub dialogue box is 0.05, which can be changed according to the need.
Click Options… push button. Options sub dialogue box will be opened. Select the Descriptive and Homogenity of variance test check box and see that all the settings remain as shown in the figure below. Click Continue to close this sub dialogue box and come back to One-Way ANOVA dialogue box. Click OK to see the output viewer.
The Output:
ONEWAY Score BY City
/STATISTICS DESCRIPTIVES HOMOGENEITY
/MISSING ANALYSIS
/POSTHOC=TUKEY ALPHA(0.05).
Descriptives
Student Score
N
Mean
Std. Deviation
Std. Error
95% Confidence Interval for Mean
Minimum
Maximum
Lower Bound
Upper Bound
Delhi
20
447.3500
104.69016
23.40943
398.3535
496.3465
269.00
599.00
Kolkata
20
437.8500
79.75771
17.83437
400.5222
475.1778
300.00
599.00
Mumbai
20
387.4000
67.25396
15.03844
355.9242
418.8758
250.00
498.00
Chennai
20
377.7000
68.49287
15.31547
345.6443
409.7557
259.00
498.00
Total
80
412.5750
85.54676
9.56442
393.5375
431.6125
250.00
599.00
Test of Homogeneity of Variances
Student Score
Levene Statistic
df1
df2
Sig.
2.371
3
76
.077
Since, homogeneity of variance should not be there for conducting Anova tests, which is one of the assumptions of Anova, we see that Levene’s test shows that homogeneity of variance is not significant (p>0.05). As such, you can be confident that population variances for each group are approximately equal. We can see the Anova results ahead.
ANOVA
Student Score
Sum of Squares
df
Mean Square
F
Sig.
Between Groups
73963.450
3
24654.483
3.716
.015
Within Groups
504178.100
76
6633.922
Total
578141.550
79
Table above shows the F test values along with degrees of freedom (2,76) and significance of 0.15. Given that p<.05, you can reject the null hypothesis and accept the alternate hypothesis that there is significant difference in scores from different metro cities of India, F(3,76)=3.716, p<.05.
Multiple Comparisons
Student Score
Tukey HSD
(I) Metro City
(J) Metro City
Mean Difference (I-J)
Std. Error
Sig.
95% Confidence Interval
Lower Bound
Upper Bound
Delhi
Kolkata
9.50000
25.75640
.983
-58.1568
77.1568
Mumbai
59.95000
25.75640
.101
-7.7068
127.6068
Chennai
69.65000*
25.75640
.041
1.9932
137.3068
Kolkata
Delhi
-9.50000
25.75640
.983
-77.1568
58.1568
Mumbai
50.45000
25.75640
.213
-17.2068
118.1068
Chennai
60.15000
25.75640
.099
-7.5068
127.8068
Mumbai
Delhi
-59.95000
25.75640
.101
-127.6068
7.7068
Kolkata
-50.45000
25.75640
.213
-118.1068
17.2068
Chennai
9.70000
25.75640
.982
-57.9568
77.3568
Chennai
Delhi
-69.65000*
25.75640
.041
-137.3068
-1.9932
Kolkata
-60.15000
25.75640
.099
-127.8068
7.5068
Mumbai
-9.70000
25.75640
.982
-77.3568
57.9568
*. The mean difference is significant at the 0.05 level.
Using Tukey HSD further, we can conclude that Delhi and Chennai have significant difference in their scores. This can be concluded from figure above and figure below.
Student Score
Tukey HSDa
Metro City
N
Subset for alpha = 0.05
1
2
Chennai
20
377.7000
Mumbai
20
387.4000
387.4000
Kolkata
20
437.8500
437.8500
Delhi
20
447.3500
Sig.
.099
.101
Means for groups in homogeneous subsets are displayed.
a. Uses Harmonic Mean Sample Size = 20.000.
Working Example 2 : One-way between groups ANOVA with Brown-Forsythe and Weltch tests
Aditya wants to see that there exists a significant difference between collecting information (internet use) and internet benefits. He collects data from 29 respondents and finds the solution through one way Anova.
Note: The respondent’s count in the working example is kept small for showing all the 29 responses in data view window in figure ahead.
Null Hypothesis : There is no significant difference in collecting information and internet benefits.
Alternate Hypothesis : There is significant difference in collecting information and internet benefits.
Internet Use
Collecting Information(Info) [see figure below]
Internet Benefits
Availability of updated information(Use1)
Easy movement across websites(Use2)
Prompt online ordering(Use3)
Prompt query handling(Use4)
Get lowest price for product/service purchase(Compar1)
Easy comparison of product/service from several vendors(Compar2)
Easy comparison of price from several vendors(Compar3)
Able to obtain competitive and educational information regarding product/ service(Compar4)
Reduced order processing time(RedPTM1)
Reduced paper flow(RedPTM2)
Reduced ordering costs(RedPTM3)
Info (Collecting Information) : 1(Never), 2(Occasionally), 3(Considerably), 4(Almost Always), 5(Always)
Internet Benefits : 1(Not important), 2(Less important), 3(Important), 4(Very Important), 5(Extremely Important)
Enter the variable view of variables as shown in the figure below.
Enter the data in the data view as shown in the figure below.
Click Analyzeƒ Compare Meansƒ One-Way ANOVA…. The One-Way ANOVA dialogue box will be opened.
Insert all the internet benefits variables in dependent list and internet use variable in the factor as shown in the figure below.
Click Post Hoc… push button to open its sub dialogue box. See that significance level is set as per need. In this case, we have used 0.05 significance level. Click Continue to close the sub dialogue box.
Click Options… push button in the One-Way ANOVA dialogue box. Select the Descriptive, Homogeneity of variance test, Brown-Forsythe and Welch check boxes and click continue to close this sub dialogue box. Click OK to see the output viewer.
The OUTPUT
ONEWAY Use1 Use2 Use3 Use4 Compar1 Compar2 Compar3 Compar4 RedPTM1 RedPTM2 RedPTM3 BY InfoG2
/STATISTICS HOMOGENEITY BROWNFORSYTHE WELCH
/MISSING ANALYSIS.
Test of Homogeneity of Variances
Levene Statistic
df1
df2
Sig.
Availability of Updated information
1.117
3
25
.361
Easy Movement across around websites
.475
3
25
.703
Prompt online ordering
.914
3
25
.448
Prompt Query handling
2.379
3
25
.094
Get lowest price for product / service purchase
1.327
3
25
.288
Easy comparison of product / service from several vendors
.755
3
25
.530
Easy comparison of price from several vendors
3.677
3
25
.025
Able to obtain competitive and educational information regarding product / service
1.939
3
25
.149
Reduced order processing time
.326
3
25
.806
Reduced Paper Flow
1.478
3
25
.245
Reduced Ordering Costs
2.976
3
25
.051
Table above shows that Easy comparison of price from several vendors has significantly different variances according to levene statistic and showing significant level of only 0.025 (which is below 0.05 for 5% level of significance) as such anova result may not be valid for this variable. Therefore, Brown-Forsythe and Welch tests are performed for analyzing this particular variable.
ANOVA
Sum of Squares
df
Mean Square
F
Sig.
Availability of Updated information
Between Groups
.702
3
.234
1.775
.178
Within Groups
3.298
25
.132
Total
4.000
28
Easy Movement across around websites
Between Groups
2.630
3
.877
1.817
.170
Within Groups
12.060
25
.482
Total
14.690
28
Prompt online ordering
Between Groups
1.785
3
.595
2.154
.119
Within Groups
6.905
25
.276
Total
8.690
28
Prompt Query handling
Between Groups
1.742
3
.581
2.132
.121
Within Groups
6.810
25
.272
Total
8.552
28
Get lowest price for product / service purchase
Between Groups
.059
3
.020
.074
.974
Within Groups
6.631
25
.265
Total
6.690
28
Easy comparison of product / service from several vendors
Between Groups
.604
3
.201
.617
.610
Within Groups
8.155
25
.326
Total
8.759
28
Easy comparison of price from several vendors
Between Groups
6.630
3
2.210
4.582
.011
Within Groups
12.060
25
.482
Total
18.690
28
Able to obtain competitive and educational information regarding product / service
Between Groups
1.302
3
.434
2.212
.112
Within Groups
4.905
25
.196
Total
6.207
28
Reduced order processing time
Between Groups
.273
3
.091
.259
.854
Within Groups
8.762
25
.350
Total
9.034
28
Reduced Paper Flow
Between Groups
.140
3
.047
.110
.954
Within Groups
10.619
25
.425
Total
10.759
28
Reduced Ordering Costs
Between Groups
.647
3
.216
.453
.718
Within Groups
11.905
25
.476
Total
12.552
28
Table above shows the F test values along with significance in case of collecting information (Internet use). Comparing the F test values and significance values, we see that all the anova comparisons favour the acceptance of null hypothesis. Please note that significance values are greater than 0.05 in all the variables except easy comparison of price from several vendors, according to homogeneity rule, this variable will not be judged by Anova F statistic. For this variable, we have performed Welch and Brown-Forsythe tests.
Robust Tests of Equality of Meansb,c,d
Statistica
df1
df2
Sig.
Availability of Updated information
Welch
1.123
3
7.172
.401
Brown-Forsythe
1.244
3
6.530
.368
Easy Movement across around websites
Welch
1.659
3
8.402
.249
Brown-Forsythe
2.051
3
17.509
.144
Prompt online ordering
Welch
1.633
3
7.896
.258
Brown-Forsythe
2.178
3
11.593
.145
Prompt Query handling
Welch
.
.
.
.
Brown-Forsythe
.
.
.
.
Get lowest price for product / service purchase
Welch
.
.
.
.
Brown-Forsythe
.
.
.
.
Easy comparison of product / service from several vendors
Welch
.560
3
8.014
.656
Brown-Forsythe
.682
3
12.935
.579
Easy comparison of price from several vendors
Welch
.
.
.
.
Brown-Forsythe
.
.
.
.
Able to obtain competitive and educational information regarding product / service
Welch
1.472
3
7.457
.298
Brown-Forsythe
1.827
3
9.211
.211
Reduced order processing time
Welch
.219
3
8.155
.881
Brown-Forsythe
.278
3
14.596
.840
Reduced Paper Flow
Welch
.119
3
8.021
.946
Brown-Forsythe
.122
3
15.144
.946
Reduced Ordering Costs
Welch
.735
3
8.066
.560
Brown-Forsythe
.525
3
16.006
.671
a. Asymptotically F distributed.
b. Robust tests of equality of means cannot be performed for Prompt Query handling because at least one group has 0 variance.
c. Robust tests of equality of means cannot be performed for Get lowest price for product / service purchase because at least one group has 0 variance.
d. Robust tests of equality of means cannot be performed for Easy comparision of price from several vendors because at least one group has 0 variance.
Table above shows the Welch and Brown-Forsythe tests performed on the internet benefits and particularly help in analyzing easy comparison of product / service from several vendors. The significance values are much higher then required 0.05. The Statistics and significance values indicate the acceptance of null hypothesis.
The analysis and conclusion from output:
Â
Homogeneity of Variance test
Anova test
Brown-Forsythe test
Welch test
Accept Null Hypothesis
Use1
ƒ¼
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ƒ¼
Use2
ƒ¼
ƒ¼
Â
Â
ƒ¼
Use3
ƒ¼
ƒ¼
ƒ¼
Use4
ƒ¼
ƒ¼
ƒ¼
Compar1
ƒ¼
ƒ¼
Â
Â
ƒ¼
Compar2
x
x
ƒ¼
ƒ¼
ƒ¼
Compar3
ƒ¼
ƒ¼
Â
Â
ƒ¼
Compar4
ƒ¼
ƒ¼
ƒ¼
RedPTM1
ƒ¼
ƒ¼
Â
Â
ƒ¼
RedPTM2
ƒ¼
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Â
Â
ƒ¼
RedPTM3
ƒ¼
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All the results verify the Null Hypothesis acceptance. Hence, we accept null hypothesis, i.e., There is no significant difference in collecting information and internet benefits.
Working Example 3 : One-way between groups ANOVA with planned comparisons
Ritu Gupta wants to know the sales in four different metro cities of India in Diwali season. She assumes the sales contrast of 2:1:-1:-2 for Delhi:Kolkata:Mumbai:Chennai, respectively. She collects sales data from 10 respondents each from the four metro cities, collecting a total of 40 sales data.
Open new data file and make variables as shown in the figure below. The values column in the city row consists of following values:
1 – Delhi
2 – Kolkata
3 – Mumbai
4 – Chennai
Enter the sales data of 40 respondents as shown below:
City Sales (Rs. Lacs)
1 500.00
1 498.00
1 478.00
1 499.00
1 450.00
1 428.00
1 500.00
1 498.00
1 486.00
1 469.00
2 500.00
2 428.00
2 439.00
2 389.00
2 379.00
2 498.00
2 469.00
2 428.00
2 412.00
2 410.00
3 421.00
3 410.00
3 389.00
3 359.00
3 369.00
3 359.00
3 349.00
3 349.00
3 359.00
3 400.00
4 289.00
4 269.00
4 259.00
4 299.00
4 389.00
4 349.00
4 350.00
4 301.00
4 297.00
4 279.00
Click Analyzeƒ Compare Meansƒ One-Way ANOVA…. This will open One-Way ANOVA dialogue box.
Shift the Sales variable to Dependent List and City variable to Factor column.
Click Contrasts… push button to open its sub dialogue box. Enter the coefficients as shown in the figure below. Notice that the coefficient total should be zero. Click continue to close the sub dialogue box and come back to previous dialogue box.
Click Post Hoc… push button to check the significance level in the Post Hoc sub dialogue box. In this case it is 0.05. Click continue to close this sub dialogue box.
Click Options… push button to open its sub dialogue box. Select descriptive and homogeneity of variance test and click continue to close this sub dialogue box. This will open previous dialogue box. Click OK to see the output viewer.
The Output:
ONEWAY Sales BY City
/CONTRAST=2 1 -1 -2
/STATISTICS DESCRIPTIVES HOMOGENEITY
/MISSING ANALYSIS.
Descriptives
Sales (Rs.Lacs)
N
Mean
Std. Deviation
Std. Error
95% Confidence Interval for Mean
Minimum
Maximum
Lower Bound
Upper Bound
Delhi
10
480.6000
24.87837
7.86723
462.8031
498.3969
428.00
500.00
Kolkata
10
435.2000
41.99153
13.27889
405.1611
465.2389
379.00
500.00
Mumbai
10
376.4000
26.45415
8.36554
357.4758
395.3242
349.00
421.00
Chennai
10
308.1000
41.33992
13.07283
278.5272
337.6728
259.00
389.00
Total
40
400.0750
73.46703
11.61616
376.5791
423.5709
259.00
500.00
Test of Homogeneity of Variances
Sales (Rs.Lacs)
Levene Statistic
df1
df2
Sig.
1.377
3
36
.265
The Levene test statistic shows that p>.05. As such, assumption of ANOVA for homogeneity of variance has not been violated.
ANOVA
Sales (Rs.Lacs)
Sum of Squares
df
Mean Square
F
Sig.
Between Groups
167379.475
3
55793.158
46.581
.000
Within Groups
43119.300
36
1197.758
Total
210498.775
39
The Anova F-ratio and significance values suggests that season does significantly influence the sales in the cities, F(3,36) = 46.581, p<.05.
The contrast coefficients, as assumed are shown in the table below.
Contrast Coefficients
Contrast
Metro City
Delhi
Kolkata
Mumbai
Chennai
1
2
1
-1
-2
Contrast Tests
Contrast
Value of Contrast
Std. Error
t
df
Sig. (2-tailed)
Sales (Rs.Lacs)
Assume equal variances
1
403.8000
34.60865
11.668
36
.000
Does not assume equal variances
1
403.8000
34.31443
11.768
22.101
.000
Since, the assumptions of homogeneity of variance were not violated, you can discuss with assume equal variances row of upper table. The t value of 36 is highly significant (p<.05).
The descriptive table shows that during Diwali season, Delhi has maximum sales and Chennai has least sales according to the respondents. To obtain F value, the above T value will be squared, i.e. F=T2 = 11.668*11.668=136.142224. Also note that, df1 for planned comparison is always 1, i.e. df1=1 and df2 will be shown in the within groups estimate of ANOVA table above, i.e., df2=36. As such we can write the result as F(1,36)=136.142224, p<.05.
Two way ANOVA
Two way ANOVA is similar to one way ANOVA in all the aspects except that in this case additional independent variable is introduced. Each independent variable includes two or more variants.
Working Example 4 : Two way between groups ANOVA
Neha gupta wants to research that whether sales (dependent) of the respondents depend on their place(independent) and education (independent). She assigns 9 respondents from each metro city. Each respondent can select three education levels.
Place: 1(Delhi), 2(Kolkata), 3(Chennai)
Education: 1(Under graduate), 2(Graduate), 3(Post Graduate)
A total of 3x3x9 = 81 responses were collected.
She wants to know whether :
The location influences sales?
The education influences the sales?
The influence of education on sales depends on location of respondent?
Make the data file by creating variables as shown in the figure below.
Enter the data in the data view as shown in the figure below.
Click Analyzeƒ General Linear Modelƒ Univariate…. This will open Univariate dialogue box.
Choose sales and send it in dependent variable box. Similarly, choose place and education to send them in fixed factor(s) list box.
Click Options push button to open its sub dialogue box.
Click Descriptive Statistics, Estimates of effect size, Observed power and Homogeneity tests check boxes in the Display box and click continue. Previous dialogue box will open. Click OK to see the output.
The Output :
UNIANOVA Sales BY Place Education
/METHOD=SSTYPE(3)
/INTERCEPT=INCLUDE
/PRINT=ETASQ HOMOGENEITY DESCRIPTIVE OPOWER
/CRITERIA=ALPHA(.05)
/DESIGN=Place Education Place*Education.
Between-Subjects Factors
Value Label
N
Place
1
Delhi
9
2
Kolkata
9
3
Chennai
9
Education
1
Under Graduate
8
2
Graduate
12
3
Post Graduate
7
Descriptive Statistics
Dependent Variable:Sales
Place
Education
Mean
Std. Deviation
N
Delhi
Under Graduate
27.50
10.607
2
Graduate
37.60
7.829
5
Post Graduate
60.00
7.071
2
Total
40.33
13.910
9
Kolkata
Under Graduate
18.33
2.887
3
Graduate
36.33
11.846
3
Post Graduate
51.67
7.638
3
Total
35.44
16.141
9
Chennai
Under Graduate
16.67
7.638
3
Graduate
25.25
20.172
4
Post Graduate
45.00
21.213
2
Total
26.78
18.600
9
Total
Under Graduate
20.00
7.559
8
Graduate
33.17
13.901
12
Post Graduate
52.14
11.852
7
Total
34.19
16.696
27
Levene’s Test of Equality of Error Variancesa
Dependent Variable:Sales
F
df1
df2
Sig.
2.152
8
18
.084
Tests the null hypothesis that the error variance of the dependent variable is equal across groups.
a. Design: Intercept + Place + Education + Place * Education
The Levene’s test shows that homogeneity of variance assumption has not been violated, since p>.05, i.e. main effects of place and education are not significant. Therefore, neither place nor education influences the sales by respondents.
Tests of Between-Subjects Effects
Dependent Variable:Sales
Source
Type III Sum of Squares
df
Mean Square
F
Sig.
Partial Eta Squared
Noncent. Parameter
Observed Powerb
Corrected Model
4638.957a
8
579.870
4.000
.007
.640
32.004
.935
Intercept
30867.022
1
30867.022
212.948
.000
.922
212.948
1.000
Place
639.603
2
319.802
2.206
.139
.197
4.413
.391
Education
3584.455
2
1792.228
12.364
.000
.579
24.729
.988
Place * Education
92.752
4
23.188
.160
.956
.034
.640
.076
Error
2609.117
18
144.951
Total
38801.000
27
Corrected Total
7248.074
26
a. R Squared = .640 (Adjusted R Squared = .480)
b. Computed using alpha = .05
The above table shows that, education level shows significant effect on sales with p<.05, i.e. F(2,18)=12.364, p<.05. As such post-hoc analysis is required, this will be discussed ahead. The output also shows that neither place nor place & education combined have significant interaction effect on sales with both having p>.05. We can also say that, place does not influence sales, F(2,18)=2.206, p>.05. Also, place and education both does not collectively influence sales, F(3,18)=23.188, p>.05.
For Post-Hoc analysis:
Click Analyzeƒ General Linear Modelƒ Univariate…. This will open Univariate dialogue box.
Click Post-Hoc push button for opening Post-Hoc sub dialogue box. Select Education and send it in Post Hoc Tests for list box. Click Tukey check box and click continue. Click OK to see the output.
Multiple Comparisons
Sales
Tukey HSD
(I) Education
(J) Education
Mean Difference (I-J)
Std. Error
Sig.
95% Confidence Interval
Lower Bound
Upper Bound
Under Graduate
Graduate
-13.17
5.495
.068
-27.19
.86
Post Graduate
-32.14*
6.231
.000
-48.05
-16.24
Graduate
Under Graduate
13.17
5.495
.068
-.86
27.19
Post Graduate
-18.98*
5.726
.010
-33.59
-4.36
Post Graduate
Under Graduate
32.14*
6.231
.000
16.24
48.05
Graduate
18.98*
5.726
.010
4.36
33.59
Based on observed means.
The error term is Mean Square(Error) = 144.951.
*. The mean difference is significant at the .05 level.
The analysis shows that Post Graduate sales differ significantly with under graduate and graduate sales, i.e. Post Graduate and other two education levels have significant difference in sales. You can see the same results in the table below also.
Sales
Tukey HSDa,,b,,c
Education
N
Subset
1
2
Under Graduate
8
20.00
Graduate
12
33.17
Post Graduate
7
52.14
Sig.
.088
1.000
Means for groups in homogeneous subsets are displayed.
Based on observed means.
The error term is Mean Square(Error) = 144.951.
a. Uses Harmonic Mean Sample Size = 8.542.
b. The group sizes are unequal. The harmonic mean of the group sizes is used. Type I error levels are not guaranteed.
c. Alpha = .05.
With significant interaction effects, you may see simple effects and comparisons based on graphs. The discussion ahead provides brief about line graph results.
Click Graphs menuƒ Lagacy Dialogsƒ Line…. This will open Line Charts dialogue box.
Click Multiple and Define to open Define sub dialogue box.
Select other statistic radio button. By default mean will be shown, when we send sales in the box. In Category Axis send education variable and put place in define lines by box.
Click OK to see the output viewer.
The output above shows that sales increases with increase in education in all the three places.
ANCOVA (One-way Analysis of Covariance)
Analysis of covariance (ANCOVA) requires different participants to perform in each condition. As such it is suitable for only between or independent group design. ANCOVA provides an elegant means of reducing systematic bias and within groups error during the analysis. You attempt to reduce error variance due to individual differences in ANCOVA analysis. To determine whether the independent variable is indeed having an effect, the influence of en extraneous variable (covariate) on the dependent variable is statistically controlled in the analysis.
Assumptions:
Independence – scores on both the dependent variable and covariate should be independent of those scores for all other participants.
Linearity – linear relationship should exist between dependent variable and covariate for each group.
Homogeneity of regression slopes – relationship of dependent variable and covariate in each group should be same.
Normality – the dependent variables should be normally distributed with same score on the covariate and in the same group. In case scores for covariate alone are normally distributed, then ANCOVA is robust to this assumption.
Independence of covariate and treatments – In case of removing the proportion of shared variability between the dependent variable and the covariate, you should be careful that you do not also remove some of the effect of independent variable. You can measure the covariate before the beginning of experiment and avoid the above stated problem. Moreover, you may also do the same by randomly allocating participants to the different levels of independent variable.
Reliability of covariate – instrument used for analysis should be reliable.
Assumptions 1, 5, 6 relate to experimental design, assumption 4 is already discussed and assumptions 2 and 3 will be discussed in the working example further.
Working Example 5 : One Way Analysis of Covariance
Aparna wants to research, whether place has an effect on sales. She recorded the sales made by 27 respondents over a year. She knows that education also contributes in the sales. As such, she took into account this added factor as co variate. The dependent variable is sales. Independent variable is Place (1-Delhi, 2-Kolkata, 3-Chennai). The covariate is education (1-undergraduate, 2-graduate, 3-post graduate)
Make the data file by creating variables as shown in the figure below.
Enter the data in the data view as shown in the figure below.
Click Data menuƒ Split File…. This will open Split File dialogue box.
Select the organize output by groups radio button. Send place variable in the groups based on list box by clicking the right arrow button as shown in the figure below. Click OK to close the dialogue box.
The message Split by Place should appear on the status bar at the bottom right of the window, as shown in the figure below.
Now, click Graphs menuƒ Legacy Dialogsƒ Scatter /Dot…. This will open Scatter /Dot dialogue box.
Click Simple Scatter and then Click Define to open Simple Scatterplot dialogue box.
Send the Sales variable in Y-Axis and Education covariate in X-Axis list box as shown in the figure below. Click OK to see the output viewer.
The Output:
SORT CASES BY Place.
SPLIT FILE SEPARATE BY Place. GRAPH
/SCATTERPLOT(BIVAR)=Education WITH Sales
/MISSING=LISTWISE.
The figure above shows the scatterplot of Delhi respondents. The figure below shows the scatterplot of Kolkata respondents. The scatterplot shows a linear relationship between dependent variable(Sales) and covariate(Education).
The Figure below shows the scatterplot of Chennai respondents. The scatterplot shows a linear relationship between dependent variable(Sales) and covariate(Education).
The slope of regression line across all groups shows a similar pattern. As such you can proceed further with the analysis.
Before proceeding with the analysis further, kindly turn the Split file option off. Click Dataƒ Split File… to open Split file dialogue box. Select Analyze all cases, do not create groups radio button and click OK. The Split file option is now off and you can now analyze further.
Click Analyzeƒ General Linear Modelƒ Univariate…. This will open Univariate dialogue box. Send Sales variable in Dependent variable box, Place variable in Fixed factor box and Education in Covariate box as shown in the figure below.
Click Options… push button to open Options sub dialogue box. Select Descriptive Statistics, Estimates of effect size, Observed power and homogeneity tests check box in the Display option as shown in the figure below. Click Continue to close the sub dialogue box. Previous dialogue box will reappear. Click OK to see the output viewer.
The Output:
UNIANOVA Sales BY Place WITH Education
/METHOD=SSTYPE(3)
/INTERCEPT=INCLUDE
/PRINT=ETASQ HOMOGENEITY DESCRIPTIVE OPOWER
/CRITERIA=ALPHA(.05)
/DESIGN=Education Place.
Between-Subjects Factors
Value Label
N
Place
1
Delhi
9
2
Kolkata
9
3
Chennai
9
Descriptive Statistics
Dependent Variable:Sales
Place
Mean
Std. Deviation
N
Delhi
38.78
10.557
9
Kolkata
42.78
10.592
9
Chennai
42.56
14.535
9
Total
41.37
11.718
27
Levene’s Test of Equality of Error Variancesa
Dependent Variable:Sales
F
df1
df2
Sig.
1.759
2
24
.194
Tests the null hypothesis that the error variance of the dependent variable is equal across groups.
a. Design: Intercept + Education + Place
Tests of Between-Subjects Effects
Dependent Variable:Sales
Source
Type III Sum of Squares
df
Mean Square
F
Sig.
Partial Eta Squared
Noncent. Parameter
Observed Powerb
Corrected Model
884.162a
3
294.721
2.524
.083
.248
7.571
.547
Intercept
1521.681
1
1521.681
13.029
.001
.362
13.029
.933
Education
793.199
1
793.199
6.792
.016
.228
6.792
.704
Place
37.899
2
18.950
.162
.851
.014
.325
.072
Error
2686.134
23
116.788
Total
49781.000
27
Corrected Total
3570.296
26
a. R Squared = .248 (Adjusted R Squared = .150)
b. Computed using alpha = .05
The output indicates no main effect (p>0.5) for place; but a significant relationship exists between sales and education, F(2,23)=6.792, p<.05. As such, you can say that when you statistically control the education, place has no influence on the sales made.
SPSS Procedure for ANOVA
After the input data has been typed according to the variables desired according to the problem, proceed according to following steps in respective cases.
One Way ANOVA
One-way between groups ANOVA with post-hoc comparisons
Click on Analyze menuƒ Compare Meansƒ One-Way ANOVA….One-Way ANOVA dialogue box will be opened.
Select dependent variable in Dependent List box and independent variable in the Factor box.
Click Contrasts… push button. Contrasts sub dialogue box will be opened. Can select the customized settings you require. Click Continue to close this sub dialogue box and come back to One-Way ANOVA dialogue box.
Click Post Hoc… push button. Post Hoc sub dialogue box will be opened. Click Tukey test and Click Continue to close this sub dialogue box and come back to One-Way ANOVA dialogue box. Also note that significant level in this sub dialogue box is 0.05, which can be changed according to the need.
Click Options… push button. Options sub dialogue box will be opened. Select the Descriptive and Homogenity of variance test check box. Click Continue to close this sub dialogue box and come back to One-Way ANOVA dialogue box. Click OK to see the output viewer.
One-way between groups ANOVA with Brown-Forsythe and Weltch tests
Click Analyzeƒ Compare Meansƒ One-Way ANOVA…. The One-Way ANOVA dialogue box will be opened.
Insert all the dependent variables in dependent list and independent variable in the factor as shown in the figure below.
Click Post Hoc… push button to open its sub dialogue box. See that significance level is set as per need. In this case, we have used 0.05 significance level. Click Continue to close the sub dialogue box.
Click Options… push button in the One-Way ANOVA dialogue box. Select the Descriptive, Homogeneity of variance test, Brown-Forsythe and Welch check boxes and click continue to close this sub dialogue box. Click OK to see the output viewer.
One-way between groups ANOVA with planned comparisons
Click Analyzeƒ Compare Meansƒ One-Way ANOVA…. This will open One-Way ANOVA dialogue box.
Shift the dependent variable to Dependent List and independent variable to Factor column.
Click Contrasts… push button to open its sub dialogue box. Enter the coefficients (2, 1, -1, -2) one by one. Notice that the coefficient total should be zero. Click continue to close the sub dialogue box and come back to previous dialogue box.
Click Post Hoc… push button to check the significance level in the Post Hoc sub dialogue box. In this case it is 0.05. Click continue to close this sub dialogue box.
Click Options… push button to open its sub dialogue box. Select descriptive and homogeneity of variance test and click continue to close this sub dialogue box. This will open previous dialogue box. Click OK to see the output viewer.
Two Way ANOVA
Two way between groups ANOVA
Click Analyzeƒ General Linear Modelƒ Univariate…. This will open Univariate dialogue box.
Choose the dependent variable and send it in dependent variable box. Similarly, choose independent variables to send them in fixed factor(s) list box.
Click Options push button to open its sub dialogue box.
Click Descriptive Statistics, Estimates of effect size, Observed power and Homogeneity tests check boxes in the Display box and click continue. Previous dialogue box will open. Click OK to see the output.
For Post-Hoc analysis:
Click Analyzeƒ General Linear Modelƒ Univariate…. This will open Univariate dialogue box.
Click Post-Hoc push button for opening Post-Hoc sub dialogue box. Select Education and send it in Post Hoc Tests for list box. Click Tukey check box and click continue. Click OK to see the output.
With significant interaction effects, you may see simple effects and comparisons based on graphs. The discussion ahead provides brief about line graph results.
Click Graphs menuƒ Lagacy Dialogsƒ Line…. This will open Line Charts dialogue box.
Click Multiple and Define to open Define sub dialogue box.
Select other statistic radio button. By default mean will be shown, when we send sales in the box. In Category Axis send one variable and put other variable in define lines by box.
Click OK to see the output viewer
ANCOVA (One-way Analysis of Covariance)
One Way Analysis of Covariance
Click Data menuƒ Split File…. This will open Split File dialogue box.
Select the organize output by groups radio button. Send one variable in the groups based on list box by clicking the right arrow button. Click OK to close the dialogue box.
The message Split by Place should appear on the status bar at the bottom right of the window.
Now, click Graphs menuƒ Legacy Dialogsƒ Scatter /Dot…. This will open Scatter /Dot dialogue box.
Click Simple Scatter and then Click Define to open Simple Scatterplot dialogue box.
Send the dependent variable in Y-Axis and covariate in X-Axis list box as shown in the figure below. Click OK to see the output viewer.
In case the slope of regression line across all groups shows a similar pattern, you can proceed further with the analysis.
Before proceeding with the analysis further, kindly turn the Split file option off. Click Dataƒ Split File… to open Split file dialogue box. Select Analyze all cases, do not create groups radio button and click OK. The Split file option is now off and you can now analyze further.
Click Analyzeƒ General Linear Modelƒ Univariate…. This will open Univariate dialogue box. Send dependent variable in Dependent variable box, independent variable in Fixed factor box and covariate in Covariate box.
Click Options… push button to open Options sub dialogue box.
Select Descriptive Statistics, Estimates of effect size, Observed power and homogeneity tests check box in the Display option. Click Continue to close the sub dialogue box.
Previous dialogue box will reappear. Click OK to see the output viewer.
Exercise
Deepak wants to know the sales in four different cities of India in Christmas season. He assumes the sales contrast of 5:3:-4:-4 for Delhi:Banglore:Mumbai:Hyderabad, respectively. He collects sales data from 10 respondents each from the four cities, collecting a total of 40 sales data.
City Sales (Rs. Crores)
Delhi 50, 48, 47, 49, 40, 42, 50, 98, 86, 69
Banglore 40, 38, 43, 38, 39, 87, 69, 48, 41, 40
Mumbai 41, 10, 89, 39, 36, 39, 49, 29, 59, 40
Hyderabad 28, 29, 59, 99, 39, 34, 30, 31, 29, 39
Analyse through One-way between groups ANOVA with planned comparisons. Calculate F ratio alongwith Post Hoc analysis.
Mohit Rajan wants to research that whether sales (dependent) of the respondents depend on their place(independent) and age (independent). He assigns 9 respondents from each metro city. Each respondent can select from three age levels.
Place: Ram Nagar, Jyoti Colony, Vivek Vihar
Ram Nagar – 10 respondents
Jyoti Colony – 9 respondents
Vivek Vihar – 10 respondents
Age: 1(Below 25 years), 2(25-35 years), 3(above 35 years).
Place Age Sales(Rs. Lacs)
Ram Nagar 1 20, 40, 44, 35
2 30, 34, 50, 40
3 60, 55
Jyoti Colony 1 15, 25, 30
2 35,40,45,70
3 65, 80
Vivek Vihar 1 80,75,30,10
2 100, 90, 75
3 150, 89, 99
Analyse through Two-way between groups ANOVA. Calculate F ratio along with Post Hoc analysis.
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