A census is the collection of data from every member of a population. This method of data
collection is often impractical.
A representative sample is a sample in which the relevant characteristics of the sample
members are generally the same as the characteristics of the population (i.e. the sample closely
resembles the population it is drawn from).
The goal of sampling is to obtain individuals that will participate in a study so that accurate
information about the population can be obtained.
Sampling Methods
The most straightforward sample is a simple random sample. In a simple random sample, each
individual in the population has an equal chance of being selected. A certain number of
individuals from the population are chosen at random.
A sample is typically chosen without replacement. Meaning, an individual cannot be
selected more than once.
A stratified sample is one obtained by separating the population into homogeneous, nonoverlapping groups, called strata, and then obtaining a simple random sample from each stratum.
Often a stratified sample is designed to contain the same proportions of groups as the
population. For example, suppose you know that 76% of college students are classified
in-state and 24% are classified as out-of-state students. In a stratified sample of 100
students from this population, 76 in-state students would be randomly selected and
24 out-of-state students would be randomly selected.
A systematic sample is obtained by selecting every nth individual from the population. The first
individual selected is a random number between 1 and n.
How do we know what n will be? It is based on the population and sample size desired. If
the population is P, and the sample size we desire is S, then n is the nearest integer below
or equal to, P/S.
A cluster sample is obtained by selecting all individuals within a randomly selected collection
or group of individuals. More than one group can be selected; but, each member in the group
participates in the study.
A convenience sample is one in which the individuals in the sample are easily obtained.