Sample size determination
is the act of choosing the number of observations to include in a statistical sample
. The sample size is an important feature of any empirical study in which the goal is to make inferences
about a population
from a sample. In practice, the sample size used in a study is determined based on the expense of data collection, and the need to have sufficient statistical power
. In complicated studies there may be several different sample sizes involved in the study: for example, in as survey sampling
involving stratified sampling
there would be different sample sizes for each population. In a census
, data are collected on the entire population, hence the sample size is equal to the population size. In experimental design
, where a study may be divided into different treatment groups
, there may be different sample sizes for each group.
Sample sizes may chosen in several different ways:
- expedience - For example, include those items readily available or convenient to collect. A choice of small sample sizes, though sometimes necessary, can result in wide confidence intervals or risks of errors in statistical hypothesis testing.
- using a target variance for an estimate to be derived from the sample eventually obtained
- using a target for the power of a statistical test to be applied once the sample is collected.
How samples are collected is discussed in sampling
and survey data collection
Larger sample sizes generally lead to increased......