- I posted this on 8/22 to sci.stat.math. However, the original request was posted in three different groups, and the other two are both more appropriate for the topic; and have had Replies.
This is a (re)post to sci.stat.edu and sci.stat.consult. ======= as posted before On 20 Aug 2003 19:59:17 -0700, [EMAIL PROTECTED] (han) wrote: > I am very sorry to make a typo. The type 1 error is 0.05. > > The original document is as below: > > Freiman and colleagues were interested in assessing whether RCTs with > negative results had sufficient statistical power to detect a 25% and > a 50% relative difference between treatment interventions. Their > review indicated that most of the trials had low power to detect these > effects: only 7% (5/71) had at least 80% power to detect a 25% > relative change between treatment groups and that 31% (22/71) had a > 50% relative change, as statistically significant (alpha=.05, one > tailed). I don't know who Freiman is, or the area he is covering. But for Randomized Clinical Trials (RCTs) with *human* subjects, (I suspect that) those comments should be regarded as tendentious and not altogether serious. Who is making the assumption, or who is implying, that any large fraction of studies should have power to detect "minuscule" or "tiny" effects? 25% relative change, such as, 10% to 12.5%, would take a HUGE study. 100% is more believable as a current convention, if you want to use that terminology. A change of Survival rates from 20% to 40% is 100% improvement, by simple proportion. Or, it could be a change of odds from 1:4 to 2:3, so, using that better measure of Odds ratio, it is .25 to .67... something rather larger than 100%. - If Freiman is describing past studies, as he seems to be, the fractions of studies with 80% power is an indicator that the studies have *not* sought to have power for those changes. Pragmatically, they *were* treated as being too small to matter. Now, if he wants to say that there would be a cost-benefit to seek improvements that are so small as 25%, then he has an argument to make. And he has to be more detailed about what makes-up success, too. But are you going to take an expensive drug (or other treatment), with unexplored side effects, if the best-guess for its efficacy was that it should raise your chance of not-relapsing (say) from 20% to 25%? Further, as to the figuring: Would that be the same bet, as wagering on a change from 80 to 85%? -- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html "Taxes are the price we pay for civilization." Justice Holmes. . . ================================================================= Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at: . http://jse.stat.ncsu.edu/ . =================================================================
