A panel study is a type of nonexperimental longitudinal study design that samples (using random or stratified random methods) a population to identify a set of respondents, then recontacts the same respondents repeatedly over time. A traditional panel study differs from a trend study in that a trend study samples the same population repeatedly but with different respondents at each time point. A panel study is similar to a traditional epidemio-logic cohort study in that a sample or cohort is followed over time. However, the purpose of following participants in a traditional cohort study is to allow enough time to be able to observe a particular outcome and determine how exposure to a certain variable is associated with the outcome. In contrast, the purpose of following participants in a panel study is typically to observe changes on a particular variable over time to potentially relate those changes to an outcome of interest or explain what factors may lead to the changes in the variable over time. Thus, a panel study often includes multiple measurements of variables, usually at regular intervals, over a circumscribed period of time.
Compared to other nonexperimental study designs such as case-control or cross-sectional studies, a carefully conducted panel study can strengthen the case for inferring causation because temporality (i.e., the cause precedes the effect in time) is more accurately preserved. In panel studies, a researcher is also able to determine the strength of an association between an explanatory variable and an outcome. However, specificity or the degree to which a cause leads to a single effect is not as high as it is in a randomized controlled trial or experimental study. Thus, in panel studies statistical controls for group non-equivalence are often employed. A further advantage of panel studies is that they allow for complex statistical modeling of dynamic phenomena (e.g., latent growth modeling and other types of trajectory analyses) that may more accurately reflect reality.
One primary disadvantage of panel studies is the problem of selective attrition over time. Participants who remain in the panel over time may be qualitatively different from the participants who are lost to follow-up due to relocation, lack of interest, or death. Another disadvantage is that panel studies can be quite costly as they require measurements at multiple time points, and tracking highly mobile respondents can be a challenge. Panel studies are strengthened when the measurement protocol or instrument remains the same over time. However, this can be difficult to maintain because of emerging technological advances in instruments or measurements or the difficulty in measuring the same phenomena during developmen-tally different phases of life. A change in the measurement protocol or instrumentation makes it difficult to determine if an observed change in the data is the result of measurement protocol changes or of actual changes. Finally, panel surveys can also be fatiguing to participants, leading to selective attrition, or participants may become primed to the instrument or measurement protocol, leading to atypical responses. Large scale, well-conducted panel studies include the Panel Study of Income Dynamics, National Longitudinal Surveys, the Health and Retirement Survey, the National Longitudinal Study of Adolescent Health, Coronary Artery Risk Development in Young Adults, and the Framingham Heart Study.