On the Misperception of Variability

Ever since the days of Francis Bacon it has been claimed that people perceive the world as less variable and more regular than it actually is. Such misperception, if shown to exist, could explain a host of perplexing behaviors. However, the only evidence supporting the claim is indirect, and there is no explanation of its cause. As a possible cause, we suggest the use of sample variability as an estimate of population variability. This is so since the sampling distribution of sample variance is downward attenuated, the attenuation being substantial for sample sizes that people are likely to consider. The results of five experiments show that people use sample variability, uncorrected for sample size, in tasks in which a correction is normatively called for, and indeed perceive variability as smaller than it actually is.