Measuring Change Over Time
Many surveyors want to track changes over time in people’s attitudes, opinions, and behaviors. To measure change, questions are asked at two or more points in time. A cross-sectional design, the most common one used in public opinion research, surveys different people in the same population at multiple points in time. A panel or longitudinal design, frequently used in other types of social research, surveys the same people over time. For more information about interviewing the same people over time see reinterviews.
Many of the questions in Pew Research surveys have been asked in prior polls. Asking the same questions at different points in time allows us to report on changes in the overall views of the general public (or a subset of the public, such as registered voters, men or African-Americans). For example, Pew Research surveys showed that in 2008, Americans’ views of the economy shifted dramatically; people rated economic conditions more negatively over the course of the year. In December, only 7% of the public rated the economy as excellent or good, a decrease of 19 points from January when 26% said economic conditions were in excellent or good shape.
When measuring change over time, it is important to use the same question wording and be sensitive to where the question is asked in the questionnaire to maintain a similar context as when the question was asked previously (see Question wording and Question order for further information). All of our survey reports include a topline questionnaire that provides the exact question wording and sequencing, along with results from the current poll and previous polls in which the question was asked.