Stata v 12 Illustration. One Way Analysis of Variance
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1 Stata v 12 Illustration Page 1. Preliminary Download anovaplot.. 2. Descriptives Graphs. 3. Descriptives Numerical 4. Assessment of Normality.. 5. Analysis of Variance Model Estimation.. 6. Tests of Equality of Variances.. 7. Post-hoc Pairwise Comparison of Groups 8. Post-hoc Graphs Source Hulley et al (1998) Randomized trial of estrogen plus progestin for secondary prevention of heart disease in postmenopausal women. The Heart and Estrogen/progestin Replacement Study. Journal of the American Medical Association, 280(7), Source Data The actual data set contains information on n=2,763. This was a randomized controlled trial investigation of hormone therapy for the prevention of heart attack and death. Notes (1) For this illustration, I have taken a subsample of size n=612 so that students with small stata will be able to reproduce this analysis. (2) Specifically, I took a random sample of 300 whites, all 218 African-Americans, plus all 94 women of other race ethnicity. The total sample size of the data set used in this illustration is thus 612. Analysis Question In this hypothetical data set, does systolic blood pressure (sbp) vary by race-ethnicity (raceth)? Stata Data Set used in this Illustration \STATA v 12 one way anova.doc Page 1 of 16
2 Key: Green stata comments Black stata command Blue output Red annotations 1. Preliminary Download anovaplot. * Download "add-on" anova command anovaplot if you don t already have it. findit anovaplot Click here to download Input Data. * Input HERS data (small version for class). use " \STATA v 12 one way anova.doc Page 2 of 16
3 2. Descriptives - Graphs. ** Set scheme for graphs user choice. set scheme s1color. * get min and max of y=sbp for y-axis tick marks. tabstat sbp, stat(min max) variable min max sbp * retrieve correspondence between codes and code labels and get sample sizes. numlabel, add. tabulate raceth race/ethnicity Freq. Percent Cum White African American Other Total ** Side-by-side dot plot - PLAIN. sort raceth. dotplot sbp, over(raceth) sbp distributions are similar in the 3 groups. It s unlikely that anova will be significant. \STATA v 12 one way anova.doc Page 3 of 16
4 . ** Side-by-side dot plot - PRETTY. dotplot sbp, over(raceth) msymbol(d) msize(vsmall) mcolor(blue) ylabel(75(25)200, labsize(small)) xlabel(1 "Whites (n=300)" 2 "African-Americans (n=218)" 3 "Other (n=94)", labsize(small) angle(45)) title("systolic BP (mm HG), by Race-Ethnicity") subtitle("n=612") caption("dot_pretty.png", size(vsmall)) \STATA v 12 one way anova.doc Page 4 of 16
5 . ** Side-by-side box plot - PLAIN. graph box sbp, over(raceth). ** Side-by-side box plot - PRETTY. graph box sbp, over(raceth) ylabel(75(25)200, labsize(small)) title("systolic BP (mm HG), by Race-Ethnicity") subtitle("n=612") caption("box_pretty.png", size(vsmall)) \STATA v 12 one way anova.doc Page 5 of 16
6 3. Descriptives - Numerical. * Descriptives - Numerical Descriptives of Raw Data. tabstat sbp, by(raceth) stat(n mean sd sem min q max) Summary for variables: sbp by categories of: raceth (race/ethnicity) raceth N mean sd se(mean) min p25 p50 p75 max White African Ameri Other Total Numerical descriptives are telling us the same. The means range mm Hg to mm Hg. \STATA v 12 one way anova.doc Page 6 of 16
7 4. Assessment of Normality. * Shapiro-Wilk Test (NULL: Distribution is normal and is retained for pvalue=large). swilk sbp Shapiro-Wilk W test for normal data Variable Obs W V z Prob>z sbp The null hypothesis of normality is rejected (p <.00001). Need graphical look to see if we re really in trouble.. ** histogram of overlay normal - PLAIN. histogram sbp, normal (bin=24, start=95, width=4.125) \STATA v 12 one way anova.doc Page 7 of 16
8 . ** histogram of overlay normal - PRETTY. histogram sbp, normal start(70) width(5) percent xlabel(75(25)200, labsize(small)) title("distribution of Y=sbp") subtitle("assessment of Normality") caption("histogram_pretty.png", size(vsmall)) (bin=25, start=70, width=5) Not bad actually!! For purposes of this illustration, we ll proceed under the assumption of normality. \STATA v 12 one way anova.doc Page 8 of 16
9 5. Analysis of Variance Model Estimation A one way anova can be obtained with either of two commands: anova or oneway..* The command ANOVA uses deviation from means parameterization.* anova YVARIABLE FACTOR. anova sbp raceth Number of obs = 612 R-squared = Root MSE = Adj R-squared = Source Partial SS df MS F Prob > F Model raceth Residual Total As expected, the F-test for the null hypothesis of equality of means is not statistically significant (p=.32)..* The command ONEWAY uses deviation from means and provides Bartlett test of equal variances..* oneway YVARIABLE FACTOR. oneway sbp raceth Analysis of Variance Source SS df MS F Prob > F Between groups Within groups Total Bartlett's test for equal variances: chi2(2) = Prob>chi2 = * The command ONEWAY with option TABULATE. oneway sbp raceth, tabulate race/ethnic Summary of systolic blood pressure ity Mean Std. Dev. Freq White Africa Other Total Analysis of Variance Source SS df MS F Prob > F Between groups Within groups Total \STATA v 12 one way anova.doc Page 9 of 16
10 Bartlett's test for equal variances: chi2(2) = Prob>chi2 = *** Reference Cell Coding Approach requires two steps.* Step 1 Command is anova. anova sbp raceth Number of obs = 612 R-squared = Root MSE = Adj R-squared = Source Partial SS df MS F Prob > F Model raceth Residual Total * Step 2 Command is regress. regress Source SS df MS Number of obs = F( 2, 609) = 1.14 Model Prob > F = Residual R-squared = Adj R-squared = Total Root MSE = sbp Coef. Std. Err. t P>t [95% Conf. Interval] raceth _cons \STATA v 12 one way anova.doc Page 10 of 16
11 6. Tests of Equality of Variances. * BARTLETT s Test is provided in output from command oneway. * Caution: This test is sensitive to the assumption of normality. oneway sbp raceth Analysis of Variance Source SS df MS F Prob > F Between groups Within groups Total Bartlett's test for equal variances: chi2(2) = Prob>chi2 = The null hypothesis of equal variances is not rejected (Bartlett test p-value=.20). * LEVENE and BROWN-FORSYTHE tests are obtained using the command robvar. * These are good choices when assumption of normality is in question.. * W_0 = Levene test. * W_50 = Forsythe-Browne modification of Levene test (mean is replaced by median). * W_10 = Fosythe-Browne modification of Levene test (mean is replaced by 10% trim). * robar(yvar), by(factor). robvar sbp, by(raceth) race/ethnic Summary of systolic blood pressure ity Mean Std. Dev. Freq White African A Other Total W0 = df(2, 609) Pr > F = Levene W50 = df(2, 609) Pr > F = Brown-Forsythe with median W10 = df(2, 609) Pr > F = Brown-Forsythe with 10% trimmed mean The null hypothesis of equal variances is not rejected by Levene s test either (p-value=.24) \STATA v 12 one way anova.doc Page 11 of 16
12 7. Post-Hoc Pairwise Comparisons of Groups Pairwise comparisons of groups is done using the command pwcompare. Note You must have fit the model first using anova.* Be sure to have first fit model using anova. anova sbp raceth Number of obs = 612 R-squared = Root MSE = Adj R-squared = Source Partial SS df MS F Prob > F Model raceth Residual Total * No adjustment for multiple comparisons.* pwcompare FACTOR. pwcompare raceth Pairwise comparisons of marginal linear predictions Margins : asbalanced Unadjusted Contrast Std. Err. [95% Conf. Interval] raceth 2 vs vs vs For all pairwise comparisons of groups, the 95% confidence interval includes the null hypothesis value of zero. \STATA v 12 one way anova.doc Page 12 of 16
13 . * Bonferroni adjustment (NOT RECOMMENDED) for Multiple Comparisons sorted and with p-values. pwcompare raceth, mcompare(bonferroni) sort effects Pairwise comparisons of marginal linear predictions Margins : asbalanced Number of Comparisons raceth Bonferroni Bonferroni Contrast Std. Err. t P>t [95% Conf. Interval] raceth 3 vs vs vs Even with the stringent Bonferroni adjustment, for all pairwise comparisons of groups, no statistically significant differences are found. Not surprising, given what we ve already seen.. * Tukey adjustment for Multiple Comparisons. * NOTE This requires equal sample sizes in all groups. * So, technically, I should not have done this.. pwcompare raceth, mcompare(tukey) Pairwise comparisons of marginal linear predictions Margins : asbalanced Number of Comparisons raceth Tukey Contrast Std. Err. [95% Conf. Interval] raceth 2 vs vs vs \STATA v 12 one way anova.doc Page 13 of 16
14 Note: The tukey method requires balanced data for proper level coverage. A factor was found to be unbalanced. Yes, yes, I know. I should not have done the Tukey procedure 8. Post-Hoc Graphs Note You must have fit the model first using anova. anova sbp raceth Number of obs = 612 R-squared = Root MSE = Adj R-squared = Source Partial SS df MS F Prob > F Model raceth Residual Total * anova plot - PLAIN. anovaplot \STATA v 12 one way anova.doc Page 14 of 16
15 . *anovaplot - PRETTY. anovaplot, legend(off) title("one Way ANOVA of Y=sbp over X=Race-Ethnicity") subtitle("n=612") ylabel(75(25)200, labsize(small)) xlabel(1 "Whites (n=300)" 2 "African- Americans (n=218)" 3 "Other(n=94)", labsize(small) angle(45)) caption("anovaplot_pretty.png", size(vsmall)). * I think I ll drop the things I created to do Levene s test before exiting.. drop m. drop absolutediff. drop yhat \STATA v 12 one way anova.doc Page 15 of 16
16 . log close \STATA v 12 one way anova.doc Page 16 of 16
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