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From |
"Pieter-Jan" <duikerarts@home.nl> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: rmanova or anova with repeated command, what to use? |

Date |
Tue, 4 Oct 2011 21:58:04 +0200 |

Dear statalisters, Some time ago we performed a randomized crossover study in which we monitored the lung function of a group of volunteers during three days. Day 1 was used to determine baseline lung function whereas day 2 and 3 were used to monitor lung function after inhaling either placebo (air) or active gas (oxygen) under hyperbaric condition. During each measurement day lung function was measured 6 times. All variables and observations were put in a dataset which initially had the following format: ID Group Time Result1 etc 1 0 0 6.19 2 0 0 5.97 . . . . . . . . 13 2 22 5.33 Etc ID exist of 13 persons (nr 1-13) Group: 0 (baseline), 1 (placebo), 2 (active) Time: pre, 0, 2, 4, 8, 12, 22 hours after exposure We want to perform a repeated measures anova as all subject performed all three test days which makes the groups not independent. The format we had in mind was to search for differences between the groups at a specific time point and to look for a correlation between Result1, etc and variable time. I found two possibilities of doing a repeated measures anova using Stata 9.2: 1. Using Ado rmanova written by George Hoffman 2. Using anova with command repeated. I tried both commands but they gave some contradictory results. To gave an example I put in the log results of one test: . rmanova FEF50 id time group ANOVA for var FEF50 by subject ID n=198 df=32 R2=.90011879 between effect: group Source | Partial SS df MS F Prob > F -------------+---------------------------------------------------- group | 94.488177 2 47.2440885 0.17 0.8420 id*group | 8196.56138 30 273.218713 within effect: time Source | Partial SS df MS F Prob > F -----------+---------------------------------------------------- Time | 218.17681 17 12.83393 1.78 0.0343 Residual | 1226.78373 170 7.21637487 . anova FEF50 id group time, repeated(group time) Number of obs = 198 R-squared = 0.8668 Root MSE = 2.68633 Adj R-squared = 0.8457 Source | Partial SS df MS F Prob > F -----------+---------------------------------------------------- Model | 7984.27781 27 295.713993 40.98 0.0000 | Id | 7766.101 10 776.6101 107.62 0.0000 group | 45.8076387 2 22.9038193 3.17 0.0443 time | 123.688633 15 8.24590887 1.14 0.3222 | Residual | 1226.78373 170 7.21637487 -----------+---------------------------------------------------- Total | 9211.06154 197 46.7566576 Between-subjects error term: id Levels: 11 (10 df) Lowest b.s.e. variable: id Repeated variable: group Huynh-Feldt epsilon = 0.6682 Greenhouse-Geisser epsilon = 0.6227 Box's conservative epsilon = 0.5000 ------------ Prob > F ------------ Source | df F Regular H-F G-G Box -----------+---------------------------------------------------- group | 2 3.17 0.0443 0.0651 0.0685 0.0784 Residual | 170 -----------+---------------------------------------------------- Repeated variable: time Huynh-Feldt epsilon = 0.4478 Greenhouse-Geisser epsilon = 0.2469 Box's conservative epsilon = 0.0667 ------------ Prob > F ------------ Source | df F Regular H-F G-G Box -----------+---------------------------------------------------- Time | 15 1.14 0.3222 0.3460 0.3482 0.3073 Residual | 170 -----------+---------------------------------------------------- . log close In rmanova there is a significant correlation with FEF50 and time (p=0.0343), while in the anova with repeated command test there is none such correlation. Can anyone advise us which option we should use: rmanova or anova with the repeated command? Many thanks. Sincerely Yours, Pieter-Jan van Ooij * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**st: RE: rmanova or anova with repeated command, what to use?***From:*"Hoffman, George" <ghoffman@mcw.edu>

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