A longitudinal study (or longitudinal survey, or panel study) is a
research design
Research design refers to the overall strategy utilized to answer research questions. A research design typically outlines the theories and models underlying a project; the research question(s) of a project; a strategy for gathering data and info ...
that involves repeated observations of the same variables (e.g., people) over long periods of time (i.e., uses
longitudinal data). It is often a type of
observational study
In fields such as epidemiology, social sciences, psychology and statistics, an observational study draws inferences from a sample (statistics), sample to a statistical population, population where the dependent and independent variables, independ ...
, although it can also be structured as longitudinal
randomized experiment
In scientific method, science, randomized experiments are the experiments that allow the greatest reliability and validity of statistical estimates of treatment effects. Randomization-based inference is especially important in experimental design ...
.
Longitudinal studies are often used in social-personality and
clinical psychology, to study rapid fluctuations in behaviors, thoughts, and emotions from moment to moment or day to day; in
developmental psychology
Developmental psychology is the scientific study of how and why humans grow, change, and adapt across the course of their lives. Originally concerned with infants and children, the field has expanded to include adolescence, adult development ...
, to study developmental trends across the life span; and in
sociology
Sociology is the scientific study of human society that focuses on society, human social behavior, patterns of Interpersonal ties, social relationships, social interaction, and aspects of culture associated with everyday life. The term sociol ...
, to study life events throughout lifetimes or generations; and in
consumer research and
political polling to study consumer trends. The reason for this is that, unlike
cross-sectional studies, in which different individuals with the same characteristics are compared, longitudinal studies track the same people, and so the differences observed in those people are less likely to be the result of cultural differences across generations, that is, the
cohort effect. Longitudinal studies thus make observing changes more accurate and 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. Longitudinal studies allow
social scientists to distinguish short from long-term phenomena, such as
poverty
Poverty is a state or condition in which an individual lacks the financial resources and essentials for a basic standard of living. Poverty can have diverse Biophysical environmen ...
. 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.
Longitudinal studies can be
retrospective (looking back in time, thus using existing data such as medical records or claims database) or
prospective (requiring the collection of new data).
Cohort studies are one type of longitudinal study which sample a
cohort (a group of people who share a defining characteristic, typically who experienced a common event in a selected period, such as birth or graduation) and perform
cross-section observations at intervals through time. Not all longitudinal studies are cohort studies; some instead include a group of people who do not share a common event.
As opposed to observing an entire population, a panel study follows a smaller, selected group - called a 'panel'.
Advantages
When 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
experiment
An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs whe ...
s. Others say that 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 also of observing the temporal order of events.
Longitudinal studies do not require large numbers of participants (as in the examples below). Qualitative longitudinal studies may include only a handful of participants, and longitudinal pilot or feasibility studies often have fewer than 100 participants.
Disadvantages
Longitudinal studies are time-consuming and expensive.
Longitudinal studies cannot avoid an attrition effect: that is, some subjects cannot continue to participate in the study for various reasons. Under longitudinal research methods, the reduction in the research sample will bias the remaining smaller sample.
Practice effect is also one of the problems: longitudinal studies tend to be influenced because subjects repeat the same procedure many times (potentially introducing
autocorrelation), and this may cause their performance to improve or deteriorate.
Examples
See also
*
Cross-sectional study
*
Time series
In mathematics, a time series is a series of data points indexed (or listed or graphed) in time order. Most commonly, a time series is a sequence taken at successive equally spaced points in time. Thus it is a sequence of discrete-time data. ...
*
Panel analysis
*
Repeated measures design
References
External links
ESDS Longitudinal data serviceCentre for Longitudinal StudiesNational Centre for Longitudinal DataLongitudinal Study in Sociology
{{Authority control
Research methods
Epidemiological study projects
Statistical data types
Design of experiments
Cohort study methods
Nursing research
Observational study
Social research