Size Matters, Significantly

26 Aug

Achieving statistical significance is entirely a matter of sample size. In the frequentist world, we can always distinguish between two samples if we have enough data (except of course if the samples are exactly the same). On the other hand, we may fail to reject even large differences when sample sizes are small. For example, over 13 Deliberative Polls (list at the end), the correlation between the proportion of attitude indices showing significant change and size of the participant sample is .81 (rank ordered correlation is .71). This sharp correlation is suggestive evidence that average effect is roughly equal across polls (and hence power matters).

When the conservative thing to do is to the reject the null, for example, in “representativeness” analysis designed to see if the experimental sample is different from control, one may want to go for large sample sizes or say something about substantiveness of differences, or ‘adjust’ results for differences. If we don’t do that samples can look more ‘representative’ as sample size reduces. So for instance, the rank-ordered correlation between proportion significant differences between non-participants and participants, and the size of the smaller sample (participant sample), for the 13 polls is .5. The somewhat low correlation is slightly surprising. It is partly a result of the negative correlation between the size of the participant pool and average size of the differences.

Polls included: Texas Utilities: (CPL, WTU, SWEPCO, HLP, Entergy, SPS, TU, EPE), Europolis 2009, China Zeguo, UK Crime, Australia Referendum, and NIC