Which research method studies different age groups of subjects over a long period of time?

What Is a Longitudinal Study?

A longitudinal study is a type of correlational research study that involves looking at variables over an extended period of time. This research can take place over a period of weeks, months, or even years. In some cases, longitudinal studies can last several decades.

Longitudinal design is used to discover relationships between variables that are not related to various background variables. This observational research technique involves studying the same group of individuals over an extended period.

Data is first collected at the outset of the study, and may then be repeatedly gathered throughout the length of the study. Doing this allows researchers to observe how variables change over time.

For example, imagine that a group of researchers is interested in studying how exercise during middle age could affect cognitive health as people age. The researchers hypothesize that people who are more physically fit in their 40s and 50s will be less likely to experience cognitive declines in their 70s and 80s.

To test this hypothesis, the researchers recruit a group of participants who are in their mid-40s to early 50s. They collect data related to how physically fit the participants are, how often they work out, and how well they do on cognitive performance tests. Periodically over the course of the study, the researchers collect the same types of data from the participants to track activity levels and mental performance.

Longitudinal studies are usually observational in nature, and are a type of correlational research. Longitudinal research is often contrasted with cross-sectional research. While longitudinal research involves collecting data over an extended period of time, cross-sectional research involves collecting data at a single point in time.

History

One of the earliest examples of a longitudinal analysis occurred during the 17th century in what is now Canada, when King Louis XIV gathered information from his population—including age, marital status, occupation, as well as livestock and land owned. He collected this information periodically to understand the health and economic viability of his colonies.

The oldest recorded longitudinal study on growth was conducted in the 18th century by Count Philibert Gueneau de Montbeillard. He measured his son every six months and published the information in the encyclopedia "Histoire Naturelle."

The Genetic Studies of Genius (also known as the Terman Study of the Gifted), which began in 1921, is known as one of the first studies to begin during the childhood of the participants and continue into their adulthood. Psychologist Lewis Terman's goal was to examine the similarities among gifted children and disprove the common assumption at the time, which was that gifted children were "socially inept."

Types of Longitudinal Studies

There are three major types of longitudinal studies:

• Panel study: Sampling of a cross-section of individuals
• Cohort study: Selecting a group based on a specific event, such as birth, geographic location, or historical experience
• Retrospective study: Reviewing historical information such as medical records

Benefits of Longitudinal Research

A longitudinal study can provide unique insight that might not be possible any other way. This method allows researchers to look at changes over time.

Because of this, longitudinal methods are particularly useful when studying development and lifespan issues. Researchers can look at how certain things may change at different points in life and explore some of the reasons why these developmental shifts take place.

For example, consider longitudinal studies that looked at how identical twins reared together versus those reared apart differ on a variety of variables. In these types of studies, researchers tracked participants from childhood into adulthood to look at how growing up in a different environment influences personality, achievement, and other areas.

Since the participants share the same genetics, it is assumed that any differences are due to environmental factors. Researchers can then look at what the participants have in common and where they differ to see which characteristics are more strongly influenced by either genetics or experience. Note that adoption agencies no longer separate twins, so such studies are unlikely today. Longitudinal studies on twins have shifted to those within the same household.

Some longitudinal studies take place over a period of years (or even decades). Researchers can use their data to establish a sequence of events when looking at the aging process.

Potential Pitfalls

As with other types of psychology research, longitudinal studies have strengths and weaknesses. There are some important advantages to conducting longitudinal research, but there are also a number of challenges that need to be considered.

Cost

Longitudinal studies require enormous amounts of time and are often quite expensive. Because of this, these studies often have only a small group of subjects, which makes it difficult to apply the results to a larger population.

Attrition

Another problem is that participants sometimes drop out of the study, shrinking the sample size and decreasing the amount of data collected. This tendency is known as selective attrition. Participants might drop out for a number of reasons, like moving away from the area, illness, or simply losing the motivation to participate.

In some cases, this can influence the results of the longitudinal study. If the final group no longer reflects the original representative sample, attrition can threaten the validity of the experiment.

Validity refers to whether or not a test or experiment accurately measures what it claims to measure. If the final group of participants is not a representative sample, it is difficult to generalize the results to the rest of the population.

The World’s Longest-Running Longitudinal Study

Lewis Terman aimed to investigate how highly intelligent children developed into adulthood with his study, the Genetic Studies of Genius. Results from this study were still being compiled into the 2000s.

Even before the Terman Study ended, psychologists criticized Terman for his work. Terman, a proponent of eugenics, is seen today as letting his own sexism, racism, and economic prejudice influence his study. He is also viewed as drawing major conclusions from weak evidence.

However, Terman's study continues to be influential in longitudinal studies. For instance, a recent study found new information on the original Terman sample, which was that men who had skipped a grade as children, on average, went on to have higher incomes than those who didn't skip a grade.

A Word From Verywell

A longitudinal study can provide a wealth of information on a topic. While such studies can be expensive and difficult to carry out, the information obtained from such research can be very valuable. Longitudinal studies from the past continue to influence and inspire researchers and students of psychology today.

Frequently Asked Questions

• What is the difference between a longitudinal study and a cross-sectional study?

  A longitudinal study follows up with the same sample (i.e., group of people) over time, whereas a cross-sectional study examines one sample at one point in time.

• How long is a longitudinal study?

  There is no set time for how long a longitudinal study should be. It can range from a few weeks to a few decades or even longer.

• How many participants do you need for a longitudinal study?

  There is no set number of participants needed for a longitudinal study. However, a researcher needs at least one participant, of course, to be able to measure data over time. But a larger group provides more information.

Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

1. Piccinin AM, Knight JE. History of longitudinal studies of psychological aging. Encyclopedia of Geropsychology. 2017:1103-1109. doi:10.1007/978-981-287-082-7_103

2. Terman L. Study of the gifted. In: The SAGE Encyclopedia of Educational Research, Measurement, and Evaluation. 2018. doi:10.4135/9781506326139.n691

3. Sahu M, Prasuna JG. Twin studies: A unique epidemiological tool. Indian J Community Med. 2016;41(3):177-182. doi:10.4103/0970-0218.183593

4. Warne RT. An evaluation (and vindication?) of Lewis Terman: What the father of gifted education can teach the 21st century. Gifted Child Q. 2018;63(1):3-21. doi:10.1177/0016986218799433

5. Warne RT, Liu JK. Income differences among grade skippers and non-grade skippers across genders in the Terman sample, 1936–1976. Learning and Instruction. 2017;47:1-12. doi:10.1016/j.learninstruc.2016.10.004

6. Wang X, Cheng Z. Cross-sectional studies: Strengths, weaknesses, and recommendations. Chest. 2020;158(1S):S65-S71. doi:10.1016/j.chest.2020.03.012

Additional Reading

• Caruana EJ, Roman M, Hernández-Sánchez J, Solli P. Longitudinal studies. J Thorac Dis. 2015;7(11):E537-E540. doi:10.3978/j.issn.2072-1439.2015.10.63

By Kendra Cherry
Kendra Cherry, MS, is an author and educational consultant focused on helping students learn about psychology.

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