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From |
khigbee@stata.com |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Experimental design - ANOVA/GLM? - please help |

Date |
Thu, 26 Sep 2002 16:25:47 -0500 |

Ricardo Ovaldia <ovaldia@yahoo.com> asks: > I was approached by an investigator with the following > problem. He had two groups of experimental rats, 10 > diabetic and 10 non-diabetic. Each of these rats had > one liter of 10 pups (average). On each of the pups a > series of biochemicals were measured. He wants me to > compare the mean value of these biochemicals from the > pumps from diabetic moms to the pups from non-diabetic > pups. He then suggested that I do a simple t-test > comparing the means of the two pup groups. I pointed > out that the observations are not independent because > of several pups from the same liter and that the liter > effect needs to be taken into account. > > How can I set this up in Stata? Assuming I understand your setup, you have a nested ANOVA model. I ran the following commands to create some fake data that I think has a similar form as yours. clear set obs 2 gen group = _n expand 10 sort group qui gen mom = _n in 1/10 qui replace mom = mom[_n-10] in 11/20 set seed 32981 gen z = 10 + round(uniform()*4-2,1) expand z drop z bysort group mom : gen pup = _n gen y = uniform()*8 + group compress I then looked at a table of pup counts and y value means for each group (meaning diabetic & nondiabetic moms) and each mom within group. The nested ANOVA results are then shown. . table mom group, c(mean y freq) ------------------------------ | group mom | 1 2 ----------+------------------- 1 | 4.290885 5.353604 | 11 8 | 2 | 5.389515 6.748766 | 9 9 | 3 | 4.533185 6.990387 | 9 12 | 4 | 4.732775 5.633358 | 8 10 | 5 | 6.142506 4.981017 | 10 8 | 6 | 5.124625 5.217069 | 9 10 | 7 | 5.285787 5.418501 | 11 11 | 8 | 5.234306 6.799588 | 11 9 | 9 | 5.171794 6.379981 | 8 9 | 10 | 6.112025 6.664215 | 10 8 ------------------------------ . anova y group / mom|group / Number of obs = 190 R-squared = 0.1047 Root MSE = 2.40589 Adj R-squared = 0.0046 Source | Partial SS df MS F Prob > F -----------+---------------------------------------------------- Model | 115.021745 19 6.05377604 1.05 0.4119 | group | 31.203949 1 31.203949 6.80 0.0178 mom|group | 82.6298147 18 4.59054526 -----------+---------------------------------------------------- mom|group | 82.6298147 18 4.59054526 0.79 0.7065 | Residual | 984.009577 170 5.78829163 -----------+---------------------------------------------------- Total | 1099.03132 189 5.81498054 I added a little group effect into my fake data (notice the p-value of .0178). Ken Higbee khigbee@stata.com StataCorp 1-800-STATAPC * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: Experimental design - ANOVA/GLM? - please help***From:*Ricardo Ovaldia <ovaldia@yahoo.com>

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