1. 1 Analysis of Variance (ANOVA) EPP 245/298 Statistical Analysis of Laboratory Data

2. October 27, 2005EPP 245 Statistical Analysis of Laboratory Data 2 The Basic Idea The analysis of variance is a way of testing whether observed differences between groups are too large to be explained by chance variation One-way ANOVA is used when there are k ≥ 2 groups for one factor, and no other quantitative variable or classification factor.

3. October 27, 2005EPP 245 Statistical Analysis of Laboratory Data 3 ABC 91012 7914 78 9912

4. October 27, 2005EPP 245 Statistical Analysis of Laboratory Data 4 Data = Grand Mean + Row Deviations from grand mean + Cell Deviations from row mean Are the row deviations from the grand mean too big to be accounted for by the cell deviations from the row means?

5. October 27, 2005EPP 245 Statistical Analysis of Laboratory Data 5 Red cell folate data Description: 22 rows and 2 columns. data on red cell folate levels in patients receiving three different methods of ventilation during anesthesia. Format: folate a numeric vector. Folate concentration (  g/l). ventilation a factor with levels 'N2O+O2,24h': 50% nitrous oxide and 50% oxygen, continuously for 24~hours; 'N2O+O2,op': 50% nitrous oxide and 50% oxygen, only during operation; 'O2,24h': no nitrous oxide, but 35-50% oxygen for 24~hours.

6. October 27, 2005EPP 245 Statistical Analysis of Laboratory Data 6 insheet using redcell.csv summarize folate tabulate ventilation tabulate ventilation, summarize (folate) graph box folate, over (ventilation) graph export folate1.wmf oneway folate ventilation describe ventilation encode ventilation, generate(dv) Describe dv

7. October 27, 2005EPP 245 Statistical Analysis of Laboratory Data 7. summarize folate Variable | Obs Mean Std. Dev. Min Max -------------+-------------------------------------------------------- folate | 22 283.2273 51.28439 206 392. tabulate ventilation ventilation | Freq. Percent Cum. ------------+----------------------------------- N2O+O2,24h | 8 36.36 36.36 N2O+O2,op | 9 40.91 77.27 O2,24h | 5 22.73 100.00 ------------+----------------------------------- Total | 22 100.00. tabulate ventilation, summarize (folate) | Summary of folate ventilation | Mean Std. Dev. Freq. ------------+------------------------------------ N2O+O2,24h | 316.625 58.717088 8 N2O+O2,op | 256.44444 37.121797 9 O2,24h | 278 33.756481 5 ------------+------------------------------------ Total | 283.22727 51.284391 22

8. October 27, 2005EPP 245 Statistical Analysis of Laboratory Data 8

9. October 27, 2005EPP 245 Statistical Analysis of Laboratory Data 9. oneway folate ventilation Analysis of Variance Source SS df MS F Prob > F ------------------------------------------------------------------------ Between groups 15515.7664 2 7757.88321 3.71 0.0436 Within groups 39716.0972 19 2090.32091 ------------------------------------------------------------------------ Total 55231.8636 21 2630.08874 Bartlett's test for equal variances: chi2(2) = 2.0951 Prob>chi2 = 0.351

10. October 27, 2005EPP 245 Statistical Analysis of Laboratory Data 10. describe ventilation storage display value variable name type format label variable label ------------------------------------------------------------------------------- ventilation str10 %10s. encode ventilation, generate(dv). describe dv storage display value variable name type format label variable label ------------------------------------------------------------------------------- dv long %10.0g dv. anova folate dv Number of obs = 22 R-squared = 0.2809 Root MSE = 45.72 Adj R-squared = 0.2052 Source | Partial SS df MS F Prob > F -----------+---------------------------------------------------- Model | 15515.7664 2 7757.88321 3.71 0.0436 | dv | 15515.7664 2 7757.88321 3.71 0.0436 | Residual | 39716.0972 19 2090.32091 -----------+---------------------------------------------------- Total | 55231.8636 21 2630.08874

11. October 27, 2005EPP 245 Statistical Analysis of Laboratory Data 11 Two- and Multi-way ANOVA If there is more than one factor, the sum of squares can be decomposed according to each factor, and possibly according to interactions One can also have factors and quantitative variables in the same model (cf. analysis of covariance) All have similar interpretations

12. October 27, 2005EPP 245 Statistical Analysis of Laboratory Data 12 Heart rates after enalaprilat Description: 36 rows and 3 columns. data for nine patients with congestive heart failure before and shortly after administration of enalaprilat, in a balanced two-way layout. Format: hr a numeric vector. Heart rate in beats per minute. subj a factor with levels '1' to '9'. time a factor with levels '0' (before), '30', '60', and '120' (minutes after administration).

13. October 27, 2005EPP 245 Statistical Analysis of Laboratory Data 13. drop _all. insheet using heart.rate.csv (4 vars, 36 obs). anova hr subj time Number of obs = 36 R-squared = 0.9685 Root MSE = 3.5165 Adj R-squared = 0.9540 Source | Partial SS df MS F Prob > F -----------+---------------------------------------------------- Model | 9117.52778 11 828.866162 67.03 0.0000 | subj | 8966.55556 8 1120.81944 90.64 0.0000 time | 150.972222 3 50.3240741 4.07 0.0180 | Residual | 296.777778 24 12.3657407 -----------+---------------------------------------------------- Total | 9414.30556 35 268.980159