Answer
When a small sample is obtained from a large population.
Work Step by Step
Suppose you live in a city with 1,000,000 people. According to some study, 15% of the population has blue eyes. Then you extract a random sample of 20 people and you want to know the probability that exactly 2 of them have blue eyes.
There are 150,000 people with blue eyes in this city and 850,000 people without blue eyes. So, for the first person:
$P(blue~eyes~first~person)=\frac{150,000}{1,000,000}=0.15$
Suppose that the first person randomly selected has blue eyes. Now, there are 149,999 people with blue eyes and 850,000 people without blue eyes. Now, for the second person:
$P(blue~eyes~second~person)=\frac{149,999}{999,999}=0.149999\approx0.15$
We can assume that the events "first person with blue eyes" and "second person with blue eyes" are independent.
