The data we get from counting how many people entering a building in one minute is the sample of the population size. We do this since it would be tedious or not feasible to stand and count one by one how many people enter the building and so we do a good estimate with our one minute sample
Let's assume in one minute, 15 people enter the building and it takes one hour for everyone to enter the building
Population size would be 15*60 minutes= 900 people
