Longitudinal Study | Definition, Types & Examples
In longitudinal studies, researchers repeatedly collect data from participants over an extended period of time. They can be contrasted with cross-sectional research, where data are collected at a single point in time.
Because they track changes over time, longitudinal studies are helpful for identifying risk factors associated with an outcome of interest—for example, which lifestyle factors may be related to developing a medical condition.
Longitudinal research methods are used in many fields, including medicine, public health, psychology, consumer research, environmental science, and economics.
The study began in 1956 and collected data from participants roughly every 7 years. It explores how cognitive measures such as memory, reasoning, and vocabulary change as people age. Over 6,000 adults have participated in this study to date.
What is a longitudinal study?
A longitudinal study is a research method that involves collecting data repeatedly or continuously from a group of individuals. There is no set duration for a longitudinal study—one could last only a few days, but many span years or even decades.
Longitudinal studies are generally observational in nature. This means researchers collect data from participants without manipulating any variables. However, they can also be experimental. For example, medical researchers might assign patients to two different treatment groups and compare the health outcomes of these groups over time.
Longitudinal vs cross-sectional studies
Longitudinal studies are often compared to cross-sectional studies, which collect data from participants at a single point in time.
Like longitudinal studies, cross-sectional studies can be used to explore the relationship between risk factors (exposures) and outcomes, and they are also less expensive and time-consuming than longitudinal research.
Because cross-sectional studies collect data at a single point in time, they aren’t the best option for inferring cause-and-effect relationships. However, they can be used to generate hypotheses that can then be investigated through longitudinal research.
You first conduct a cross-sectional study. You ask a sample of older adults to complete a questionnaire about their lifestyle, daily activities, and health records.
You notice a strong correlation between proximity to green space and cardiac function—older adults who live near parks and gardens have fewer cardiac incidents.
To investigate whether access to green spaces influences cardiac health in older adults, you next design a longitudinal study. You follow a group of older adults over five years, collecting data on their living situation and health every month.
These longitudinal data allow you to analyze whether access to green space impacts cardiac health in older adults.
Types of longitudinal research and examples
There are many ways to conduct longitudinal research. Longitudinal studies can be prospective or retrospective. Prospective studies recruit participants and then follow them over time to collect data, whereas retrospective studies use data that have already been collected.
Some common longitudinal research methods are outlined in the following sections.
Repeated cross-sectional study
A repeated cross-sectional study combines elements of both longitudinal and cross-sectional research.
In a cross-sectional study, data are collected from individuals at a single time point. As its name suggests, a repeated cross-sectional study is a cross-sectional study that is repeated over time. However, each time, a different group from the same population is studied.
Because data are collected from the same population over time, a repeated cross-sectional study is a type of longitudinal research. Repeated cross-sectional studies are helpful for studying changes in a population over time.
Cohort study
A cohort study (or cohort panel) recruits participants based on their “exposure status” (whether they’ve experienced something of interest). This exposure status could range from when and where someone is born to whether they smoke cigarettes. This cohort is then followed over time to see if they develop one or more outcomes of interest, such as a medical condition.
Because the exposure and outcome variables are clearly separated in time, cohort studies are useful for studying cause-and-effect relationships.
Cohort studies can be conducted prospectively (people are recruited based on outcome status and followed over time) or retrospectively (longitudinal data from existing datasets are used).
The ABCD study is following its participants for 10 years and collecting data on their brain development, cognitive abilities, and behavior. The aim of this study is to understand how certain experiences (e.g., screen time, drug and alcohol consumption, and sleep) impact brain development.
Panel study
A panel study (or representative panel) follows a group of participants over time. Unlike participants in a cohort study, who share a common feature or exposure status, participants in a panel study are often recruited randomly from a target population.
Data are collected regularly from this “panel” of participants. Researchers often conduct panel studies to observe changes in people’s opinions, behaviors, or characteristics over time. Panel studies are commonly used in consumer research but are also useful in fields like sociology, political science, and health studies.
The study began with a nationally representative panel of over 5,000 American families. Data have been collected from these families and their descendants every two years. These data have been used in thousands of academic papers.
Strengths and weaknesses of longitudinal research
All research methods have their benefits and disadvantages. Consider the following strengths of a longitudinal study.
- Detect changes over time: Because longitudinal studies follow the same people over time, researchers can observe how measures of interest evolve
- Identify cause-and-effect relationships: Longitudinal studies can be used to isolate the effects of certain exposures
- Avoid recency bias: People are better at remembering their recent experiences versus things from the past. Longitudinal studies avoid a recency bias by asking participants to report their present circumstances each time data are collected.
However, there are also several weaknesses of a longitudinal study.
- Cost and time: Because longitudinal studies often collect data over years or decades, they are time-consuming and expensive.
- Retention issues: Longitudinal research is vulnerable to attrition—some participants may be unavailable or choose not to participate over time.
- Difficult to standardize data collection across multiple sites: When data are collected from multiple research centers, differences in how data are collected at each of these locations may bias results.
- Practice effects: Longitudinal research participants generally complete the same tests repeatedly. Changes in their performance may simply be caused by their familiarity with the task, rather than actual changes in whatever the test is measuring.
Frequently asked questions about longitudinal studies
- What is the difference between a cross sectional study and a longitudinal study?
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In a cross-sectional study, researchers collect data from individuals at a single point in time.
In a longitudinal study, researchers collect data from individuals repeatedly or continuously over an extended period of time (often years).
Cross-sectional studies are faster and less expensive to conduct than longitudinal studies. However, because they collect data at a single point in time, cross-sectional studies are not the best option for establishing cause-and-effect relationships.
A common practice is to conduct a cross-sectional study to generate hypotheses. You can then use this information to design a longitudinal study.
- What are common research methods in psychology?
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Most research methods in psychology are quantitative: numerical data are used to address a research question. Quantitative approaches include the following:
- Experimental research
- Quasi-experimental research
- Survey research
Some psychology research is qualitative (i.e., uses nonnumerical data to explore a research question). Qualitative research approaches include the following:
- Interviews and focus groups
- Case studies
- Survey research
- Observational research
Mixed-methods research combines qualitative and quantitative approaches.
- What is a repeated cross-sectional study?
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In a repeated cross-sectional study, the same population is studied at multiple time points. At each time point, data are collected from a different sample of the population.
A repeated cross-sectional study is a type of longitudinal study because data are collected repeatedly over a period of time.
However, as the name suggests, it also resembles a cross-sectional study. Data are obtained from each group of participants at a single time point, and this process is repeated several times.
Repeated cross-sectional studies are helpful for studying changes in a population over time.
- How long is a longitudinal study?
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There is no minimum duration for longitudinal studies. Some are only a few days or weeks long, but many last for years or even decades.
The only requirement for a study to be considered longitudinal is that data are collected repeatedly from participants.
- What are the types of longitudinal studies?
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Longitudinal studies are broadly categorized as prospective (researchers recruit participants and collect their data over the following years) or retrospective (researchers use an existing longitudinal dataset to “look back in time” at factors contributing to an outcome of interest).
Specific longitudinal study designs include the following.
- Repeated cross-sectional study: Researchers collect the same data from distinct samples of the same population over time (essentially a cross-sectional study that is repeated with different people).
- Cohort study: Participants who share one or more characteristics (or “exposures”) are followed over time to observe which outcomes they develop.
- Panel study: A group of participants, often randomly selected, is studied over time to observe changes in their attitudes or behaviors.
