Longitudinal study

A longitudinal study is a correlational research study that involves repeated observations of the same variables over long periods of time &mdash; often many decades. It is a type of observational study. Longitudinal studies are often used in psychology to study developmental trends across the life span, and in sociology to study life events throughout lifetimes or generations. The reason for this is that unlike cross-sectional studies, longitudinal studies track the same people, and therefore the differences observed in those people are less likely to be the result of cultural differences across generations. Because of this benefit, longitudinal studies make observing changes more accurate and they are applied in various other fields. In medicine, the design is used to uncover predictors of certain diseases. In advertising, the design is used to identify the changes that advertising has produced in the attitudes and behaviors of those within the target audience who have seen the advertising campaign.

Because most longitudinal studies are observational, in the sense that they observe the state of the world without manipulating it, it has been argued that they may have less power to detect causal relationships than do experiments. But because of the repeated observation at the individual level, they have more power than cross-sectional observational studies, by virtue of being able to exclude time-invariant unobserved individual differences, and by virtue of observing the temporal order of events.

Longitudinal studies allow social scientists to distinguish short from long-term phenomena, such as poverty. If the poverty rate is 10% at a point in time, this may mean that 10% of the population are always poor, or that the whole population experiences poverty for 10% of the time. It is not possible to conclude which of these possibilities is the case using one-off cross-sectional studies.

Types of longitudinal studies include cohort studies and panel studies. Cohort studies sample a cohort, defined as a group experiencing some event (typically birth) in a selected time period, and studying them at intervals through time. Panel studies sample a cross-section, and survey it at (usually regular) intervals.

A retrospective study is a longitudinal study that looks back in time. For instance a researcher may look up the medical records of previous years to look for a trend.

Examples

 * Avon Longitudinal Study of Parents and Children (ALSPAC)
 * Born in Bradford
 * 1970 British Cohort Study (BCS70)
 * British Household Panel Survey
 * Busselton Health Study
 * Canadian Longitudinal Study
 * Child Development Project
 * Children of Immigrants Longitudinal Study (CILS)
 * Dunedin Multidisciplinary Health and Development Study
 * Fragile Families and Child Wellbeing Study
 * Framingham Heart Study
 * Genetic Studies of Genius (the world's oldest and longest-running longitudinal study)
 * German_Socio-Economic_Panel
 * Health and Retirement Study
 * Household, Income and Labour Dynamics in Australia Survey
 * Human speechome project
 * Luxembourg Income Study (LIS)
 * Millennium Cohort Study (MCS)
 * Millennium Cohort Study (United States)
 * Minnesota Twin Family Study
 * National Child Development Study (NCDS)
 * National Longitudinal Study of Children and Youth (NLSCY)
 * Pacific Islands Families Study
 * Panel Study of Belgian Households
 * Rotterdam Study
 * Seattle Longitudinal Study
 * Up Series
 * WHO's Study on global AGEing and adult health (SAGE)
 * Wisconsin Longitudinal Study

Repeated cross-sectional surveys

 * General Social Survey
 * World Values Survey