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From |
VISINTAINER PAUL <VISINT@NYMC.EDU> |

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

Subject |
st: RE: Sample size for ttest - (am I wrong?) |

Date |
Wed, 19 Feb 2003 08:47:22 -0500 |

This may help. Jacob Cohen wrote a book a while back called "Statistical Power for the Behavioral Sciences" (now published by Lawrence Erlbaum Associates, 1988). He defined the term "effect size", which for the two group t-test is the difference between the means divided by the pooled standard deviation: (m1-m0)/SDp. (In other words, the size of the difference between the means relative to the SD.) He gives the ranges and his logic behind what he considers small, medium, and large effect sizes. Small EF = about .1 to .2; medium EF= about .35 to .5; and large EF = anything greater than .75. So, your pathologist is certainly only looking for "colossal" effect sizes. Cohen states (and I think we'd all agree) that the study sample size should be large enough to detect the SMALLEST effect size that is reasonably or logistically possible. If your pathologist has no clue of the size of the effect of COX2 that he is looking for, then the most prudent approach is to use the largest sample size that he has funds to cover (e.g., 20). If his primary interest is in designing a study that is inexpensive, tell him that by looking for 2 standard deviations, he only needs to run 4 animals per group. Paul -----Original Message----- From: Ricardo Ovaldia [mailto:ovaldia@yahoo.com] Sent: Tuesday, February 18, 2003 12:11 PM To: statalist@hsphsun2.harvard.edu Subject: st: Sample size for ttest - (am I wrong?) Dear all, I feel like such an idiot and I need some help on this non-Stata problem. I was in a meeting this morning with a pathologist that is submitting a proposal to conduct an experiment to compare the concentration of a biochemical (Cox-2) in two different tissue cell lines. He wants to compare mean concentrations so I suggested a t-test, after a monotone transformation if needed. He agreed and proceeded to tell me that because of monetary constrains, he could only have 20 observations per group and wants to know, what is the minimum difference that he can detect with 80% power? I told him that I need some idea of the means and SD of the measurement and he said that he did not know because this was a new method, and furthermore I did not need this information because his friend (another "statistician") told him that with 8 observations per group he could detect a difference of 1.5 standard deviations with 80% power. I felt humiliated and embarrassed. Does anyone know how to do this, or is this guy wrong? Thank you, Ricardo ===== Ricardo Ovaldia, MS Statistician Oklahoma City, OK __________________________________________________ Do you Yahoo!? Yahoo! Shopping - Send Flowers for Valentine's Day http://shopping.yahoo.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/

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