Social Science Statistics Blog: Placebo effects and the probability of assignment to active treatment

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This blog makes public the hallway conversations about social science statistical methods and analysis from the Institute for Quantitative Social Science and related research groups. Expect to see posts on trends in methodological thought, questions and comments, paper and conference announcements, applied problems needing methodological solutions, and methodological techniques seeking applied problems. Also included are summaries of papers and comments from a popular weekly research workshop held here and billed as a tour of Harvard's statistical innovations and applications with weekly stops in different disciplines. [read more...]

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15 May 2008

Placebo effects and the probability of assignment to active treatment

I just finished reading an interesting paper on placebo effects in drug trials by Anup Malani. Malani noticed that participants in high probability trials know that they more likely to get active treatment (because of informed consent prior to the trial). They have higher expectations and hence should have higher placebo effects than patients in low probability trials. Malani compares outcomes across trials with different assignment probabilities and finds evidence for placebo effects. A related finding is that the control group in high probability trials reports more side effects.

The paper discusses some potential implications of placebo effects, e.g. that patients who are optimistic about the outcome might change their behavior and hence get better even without the active drug. It makes me wonder how this might translate into non-medical settings and whether there are studies of placebo effects in the social sciences. Also, if placebo drugs can improve health outcomes, maybe ineffective social programs would still work as long as participants don’t know whether the program works or doesn’t? Maybe this is the role of politics. But what about the side-effects?

Malani, A (2006) “Identifying Placebo Effects with Data from Clinical Trials” Journal of Political Economy, Vol. 114, pp. 236-256. http://papers.ssrn.com/sol3/papers.cfm?abstract_id=901838

Abstract:
A medical treatment is said to have placebo effects if patients who are optimistic about the treatment respond better to the treatment. This paper proposes a simple test for placebo effects. Instead of comparing the treatment and control arms of a single trial, one should compare the treatment arms of two trials with different probabilities of assignment to treatment. If there are placebo effects, patients in the higher-probability trial will experience better outcomes simply because they believe that there is a greater chance of receiving treatment. This paper finds evidence of placebo effects in trials of antiulcer and cholesterol-lowering drugs.

Posted by Sebastian Bauhoff at May 15, 2008 12:00 PM

Comments

Cool! You should also check out the paper by Jin Hui and Don Rubin in the March '08 JASA -- it deals with placebo effects as well, in the context of principal stratification and non-compliance.

Also, does anyone else find it odd that this paper was published in the Journal of Political Economy and not, say, in a mainstream medical or statistics journal?

Posted by: John Graves at May 15, 2008 12:32 PM

Thanks for the Hui and Rubin reference!

The JPE publishes an interesting mix of papers, almost freakonomic-ish (to no surprise, since Steven Levitt is one of the editors...). And Anup Malani is ``not even'' an economist but a law professor in Chicago.

BTW he has another paper of interest to policy wonks "Regulation with Placebo Effects". See here for the reference, and here for the working paper.

Posted by: Seb at May 15, 2008 2:44 PM

Interesting paper. However, remember that there are two problems. First, high- and low-probability placebo trials are not randomly assigned. Second, the assignment is known to both the researchers and the subjects. Together, this opens the threat that the effect isn't about the probability itself but about the kind of drugs that are involved in high or low probability trials. I don't know enough about medical trials to speculate, but it would seem that cost of treatment and likelihood of working would be key factors in determining the placebo rate that might also be associated with the ability of subjects to induce an unreal placebo effect.

Posted by: Michael Weiksner at May 17, 2008 4:47 PM

Michael, i agree with this comment. To really tease out this effect you need to randomly assign high- and low-probability placebo trials. I would not be surprised if the same effects show up when you actually conduct such an experiment, but it would be important to check before one jumps to conclusions.

Jens

Posted by: Jens at May 17, 2008 7:04 PM

I agree with this comment:

"Second, the assignment is known to both the researchers and the subjects. Together, this opens the threat that the effect isn't about the probability itself but about the kind of drugs that are involved in high or low probability trials."

Posted by: Anonymous at May 19, 2008 11:48 AM

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Placebo effects and the probability of assignment to active treatment

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