In statistics, a selection effect exists when some property of a thing is correlated with its being sampled. The classic example is a phone poll, which necessarily only reaches people who have phones. If only wealthy people have phones, then being wealthy is correlated with ever being polled in the first place. Thus the opinions of wealthy people will be overrepresented.
An observation selection effect exists when some property of a thing is correlated with the observer existing in the first place. The study of such effects is sometimes called "anthropic reasoning" or "anthropics", after the anthropic principle.
For example, if intelligence hadn't evolved, we wouldn't exist, and couldn't evaluate the probability of intelligence evolving. In a big enough universe, intelligence could evolve somewhere even if the probability of it happening was arbitrarily low. Therefore we cannot just infer that because intelligence evolved here, evolution is common, or that designing intelligence is easy.
Recent approaches to such effects have focused less on "anthropic principles" and more on candidate assumptions such as:
Such assumptions are needed to determine how we choose between theories predicting different sets of observers.