Population

The population is the complete set about which conclusions are to be drawn. It might be all adults in a country, all customers of a company, all transactions in a year or all shares listed on a stock exchange.

When it is possible to collect data on every member of the population, this is called a census. More often, researchers study a sample and use it to estimate population characteristics, such as the population mean or the proportion of people with a certain view.

For example, if an auditor wants to know whether a company's invoices are accurate, the population is all the invoices issued during the year. Checking every invoice may be impractical, so the auditor examines a sample.

Defining the population clearly is important. A study of "investor behaviour" needs to state whether it means all adults, all people with investment accounts or only active traders, because conclusions about one group may not apply to another. Statistical formulas also differ slightly depending on whether data cover a whole population or a sample, for example when calculating variance and standard deviation.