The term population refers to all of the measurements of interest. For example, if you were trying to find the average weight of United Airline passengers flying from San Francisco to Hong Kong for the month of March, the population would include all of the weights for all of the passengers who flew from San Francisco to Hong Kong for the month of March via United Airlines.
Use: Having 100% of the data is certainly useful for the team, but more often than not, it is either virtually impossible to get, or the data is too expensive to gather, or the team chooses not to expend the energy on collecting all of the population data. Instead, samples are generally used to make inferences about the population based upon a much smaller sample set.
