Editing Survey methodology (section)

===Successive independent samples studies===
A successive independent samples design draws multiple random samples from a population at one or more times.<ref name="Shaughnessy2011"/>  This design can study changes within a population, but not changes within individuals because the same individuals are not surveyed more than once. Such studies cannot, therefore, identify the causes of change over time necessarily. For successive independent samples designs to be effective, the samples must be drawn from the same population, and must be equally representative of it. If the samples are not comparable, the changes between samples may be due to demographic characteristics rather than time. In addition, the questions must be asked in the same way so that responses can be compared directly.