Because Web surveys are often convenience samples, traditional methods for statistical inference do not readily apply. Propensity scoring is one attempt to correct for the selection bias of a nonrandom Web sample. Broadly speaking, the propensity scoring adjustment is accomplished by reweighting the convenience sample such that the distribution of so-called propensity variables corresponds to that of a reference sample. Propensity variables [or propensity questions) can be any survey questions that have been answered both by the Web survey participants and by the respondents of the reference sample. The reference sample is a separate probability sample (e.g. a random-digit dialed [RDD] phone survey) from a possibly much shorter survey that contains only the propensity questions.