Survey research is the most commonly used data collection technique in sociology and is also used in disciplines such as political science, social psychology, economics, marketing, and communications. The unit of analysis in survey research is usually the individual, although it might be another entity such as an organization. Social scientists often use surveys to describe attitudes and behaviors of a sample. Although some survey research is solely for descriptive purposes, more often the aim is explanatory. Survey research presents various other challenges to researchers. One is connected with secondary analysis, when researchers analyze survey data that were collected by someone else at another time and typically for a different (or at least slightly different) purpose. Secondary analysis is increasingly popular, due to the ever-expanding number of state, regional, and national survey databases that are available and in light of the significant costs associated with conducting research using probability samples.