Insensitivity to Sample Size
Sample Size Neglect
The cognitive bias that occurs when people judge the probability of obtaining a sample statistic without respect to the sample size. In other words, variation is more likely in smaller samples, but people may not expect this.
Origin
Amos Tversky and Daniel Kahneman identified this bias in their landmark 1974 paper "Judgment under Uncertainty: Heuristics and Biases" in Science. They showed that subjects assigned the same probability to obtaining an extreme sample mean regardless of whether the sample contained 10, 100, or 1,000 cases — a failure rooted in the representativeness heuristic, where people expect small samples to mirror the population just as closely as large ones.
Everyday Use
A restaurant has five reviews, all five stars. Another has 500 reviews averaging 4.5 stars. Your gut says the five-star place is better — but five reviews tell you almost nothing. We routinely trust tiny samples as if they were as reliable as large ones, whether we're judging doctors, schools, or investment returns.