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From |
"Yulia Marchenko" <ymarchenko@stata.com> |

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

Subject |
st: RE: comparing mean |

Date |
Mon, 9 Aug 2004 10:21:32 -0500 |

On Monday, Samir wrote: >Dear Statalisters, >I am comparing average change in a utility score of patients in two >groups. I see that the initial distribution of utility score is not >same, one group have more patients with higher severity than the other. >How can I compare the mean between these two groups controlling for the >dissimilar distribution? How can I do it in Stata? Before deciding what type of test to use you need to look at the distribution of scores for each group. If it is fairly "bell-shaped" and does not have severe outliers the usual two-sample t-test to compare means can be used. Stata has -ttest- command to perform this test. For example, . sysuse auto, clear . ttest price, by(foreign) The difference in distributions might be attributed not only to differences in means but also to differences in variances of each group. In this case -ttest- with option -unequal- should be used to account for different variability among groups. For example, . sysuse auto, clear . ttest price, by(foreign) unequal The Stata command -robvar- allows you to test if the variances among several groups are equal. For the previous example, . sysuse auto, clear . robvar price, by(foreign) For this particular example there is no significant evidence to reject hypothesis of equal variability among Domestic and Foreign groups. Therefore, there is no need to specify -unequal- option in -ttest-. If the distribution is heavily skewed then test on medians should be performed instead. Various commands are implemented in Stata to do this. See . help signrank For more information on the above commands, please, see Stata manuals or type . help ttest . help robvar --Yulia ymarchenko@stata.com * * 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/ * * 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/

**References**:**st: comparing mean***From:*samirkc <samir.kc@bmg.eur.nl>

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