(This article was originally published at Statistical Modeling, Causal Inference, and Social Science, and syndicated at StatsBlogs.)

Alexis Le Nestour writes:

How do you test for no effect? I attended a seminar where the person assumed that a non significant difference between groups implied an absence of effect. In that case, the researcher needed to show that two groups were similar before being hit by a shock conditional on some observable variables. The assumption was that the two groups were similar and that the shock was random. What would be the good way to set up a test in that case?

I know you’ve been through that before (http://andrewgelman.com/2009/02/not_statistical/) and there are interesting comments but I wanted to have your opinion on that.

My reply: I think you have to get quantitative here. How similar is similar? Don’t let your standard errors drive your research agenda. Or, to put it another way, what would you do if you had all the data? If your sample size were 1 zillion, then everything would statistically distinguishable from everything else. And then you’d have to think about what you really care about.

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