The word population or statistical population is used for all the individuals or objects which we must study. We may be interested in learning about the quality of bulbs produced in a factory. All the products of the factory in a certain period is called a population. We may be interested in the level of education in primary schools. All the children in the primary schools will make up a population. The population may contain living or non-living things. The entire lot of anything under study is called a population. All the fruit trees in a garden, all the patients in a hospital and all the cattle in a herd are examples of populations in different studies.
A population is called finite if it is possible to count its individuals. It may also be called a countable population. The number of vehicles crossing a bridge every day, the number of births per years and the number of words in a book are finite populations. The number of units in a finite population is denoted by . Thus is the size of the population.
Sometimes it is not possible to count the units contained in the population. Such a population is called infinite or uncountable. Let us suppose that we want to examine whether a coin is fair or not. We shall toss it a very large number of times to observe the number of heads. All the tosses will make an infinite or uncountable infinite population. The number of germs in the body of a sick patient is perhaps something which is uncountable.
Target and Sampled Population
Suppose we have to perform a study about the problems experienced by families living in rented houses in a certain big city. All the families living in rented houses are our target population. The entire target population may not be considered for the purpose of selecting a sample. Some families may not be interested in being included in the sample. We may ignore some part of the target population to reduce the cost of study. The population out of which the sample is selected is called the sampled population or studied population.