On 10 Mar 2010 at 22:15, Jim Clark wrote, referring to the claim that anti-depressants may do poorly in placebo trials because the placebo effect is getting stronger.
> I'm not sure it does make sense. If the placebo effect is getting stronger, > then shouldn't the med effect > also get stronger because of its placebo effect? That is, med effect is > hypothesized to be combination of > placebo + med, hence the rationale for the placebo control group. So you > have something like: > > placebo(1990) + med versus placebo(1990) in 1990 > > and > > placebo(2010) + med versus placebo(2010) in 2010 > > I do not see how this can lead to a narrowing of the med effect? Interesting point. As Jim notes, the overall therapeutic effect of a medication consists of two parts: a true drug effect plus its placebo effect. So if the placebo is getting stronger, then the overall medication therapeutic effect ( = drug + placebo) should also get stronger. Perhaps the paradox can be resolved in one of two ways. First, if the true drug effect remains constant over time, but the placebo contribution has increased, then placebo now makes up a greater proportion of the overall therapeutic effect. So the difference between medication and placebo conditions would get smaller. e.g., in arbitrary units: Before: overall therapeutic effect [OTE] (10) = drug (5) + placebo (5) placebo effect (5) vs OTE (10) = 50% of OTE After: OTE (15) = drug (5) + placebo (10) placebo effect (10) vs OTE (15) = 67% of OTE So the difference between placebo and OTE has decreased over time. However, this would also imply, as noted above, that OTE has also increased, and the drug is now more effective than initially (when compared with no drug, not with placebo). I wonder if there are any data to support this. Second, what if the OTE remains constant but the proportion contributed by placebo increases? This would imply that the true drug effect declines over time. In this case: Before OTE (10) = drug (5) + placebo (5) placebo effect (5) vs OTE (10) = 50% of OTE After OTE (10) = drug (3) + placebo (7) placebo effect (7) vs OTE (10) = 70% of OTE So again the difference between placebo and OTE would decrease over time. In both scenarios of increased placebo action over time, the difference in effectiveness between the drug and placebo conditions would diminish, and thus the likelihood of finding a significant difference in a drug trial. Right? Stephen -------------------------------------------- Stephen L. Black, Ph.D. Professor of Psychology, Emeritus Bishop's University e-mail: sblack at ubishops.ca 2600 College St. Sherbrooke QC J1M 1Z7 Canada ----------------------------------------------------------------------- --- You are currently subscribed to tips as: [email protected]. To unsubscribe click here: http://fsulist.frostburg.edu/u?id=13090.68da6e6e5325aa33287ff385b70df5d5&n=T&l=tips&o=1210 or send a blank email to leave-1210-13090.68da6e6e5325aa33287ff385b70df...@fsulist.frostburg.edu
