The most accurate way to study a population is to count all individuals within the population. However, this often requires tremendous time, money and resources and is rarely possible. For example, its impossible to count all the Black Swallowtail Butterflies that live in Yellowstone National Park (USA). Instead of counting an entire population, scientists typically study a portion of a population by sampling, which involves counting individuals within a certain area (or volume for aquatic organisms) that is part of their natural habitat