Law of Small Numbers
It’s the mistake to believe that a small sample accurately resembles the parent population. In reality, small samples have a small confidence interval and will eventually regress to the mean as the sample grows.
Google reviews
“Let’s try this restaurant, it’s got a 4.8 rating, higher than the other option, at 4.5”. However, with only 5 reviews the Confidence Interval of a review rating might be 4.8 ± 0.90 (hence between 3.90 and 5.00), whereas at 100 reviews 4.5 ± 0.15 (between 4.65 and 4.35). In other words, the latter restaurant’s rating is statistically more reliable.
Stock performance
Taking into account the performance of a stock over a limited period of time, rather than a longer (and more representative) projectory.
Also relates to: Representativeness Heuristic · Gambler’s Fallacy · Hot Hand Fallacy · Extrapolation Bias · Base Rate Neglect