A population is the entire collection of individual units, which can be people or measurements, about which information is desired.

Example 1

A study was conducted to examine snoring, witnessed sleep apnea, teeth grinding, primary and secondary nocturnal enuresis and sleep duration in Hong Kong primary school children (Ng et al., 2005). Suppose you are particularly interested in habitual snoring and it was reported that 333 out of 3047 respondent children had habitual snoring. The population consists of information on habitual snoring of all Hong Kong primary school children.

Example 2

A study was conducted to compare the effects of a modified day treatment program with those of traditional outpatient treatment in patients with eating disorders (Kong, 2005). A total of 21 volunteers from an outpatient clinic for eating disorders were treated with the modified day treatment program while 22 received the traditional treatment.

There are two populations. Population 1 comprises all patients with eating disorders who received the modified day treatment program. Population 2 comprises all patients with eating disorders who received the traditional treatment.