Selection refers to a situation in which data are not representative of the underlying population of interest. In particular, selection occurs when unobserved factors that determine whether an observation is in the data set also help determine the value of the quantity of interest. For example, in a study of willingness to volunteer, it would make little sense to only select those participants who volunteer to participate. An analysis of data that suffer from selection generally produces biased estimates and renders statistical inference problematic. Solutions to the problem commonly involve modeling the selection process and the outcome of interest at the same time to account for how selection influences the observed values.