Law of large numbers

principle which determines that as the trial group grows larger the results will get more statistically reliable (in Probability)

The law of large numbers (LLN) is a theorem in probability that describes the long-term stability of a random variable. Given a sample of independent and identically distributed random variables with a finite population mean and variance, the average of these observations will eventually approach and stay close to the population mean.
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Noun
1. (statistics) law stating that a large number of items taken at random from a population will (on the average) have the population statistics
(synonym) Bernoulli's law
(hypernym) law, law of nature
(classification) statistics

The mean of a random sample approaches the mean (expected value) of the population as the sample grows.