Which research method will be used when studying the same samples of participants over an extended period of time?

Longitudinal studies differ from one-off, or cross-sectional, studies. The main difference is that cross-sectional studies interview a fresh sample of people each time they are carried out, whereas longitudinal studies follow the same sample of people over time.

Features of longitudinal vs cross-sectional studies

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Some cross-sectional studies take place regularly, each time including a large number of repeat questions. For example, the British Social Attitudes Survey is a repeat cross-sectional study that has been carried out nearly every year since 1983. It provides excellent data about how Britain’s attitudes and values have changed (or not changed) over time.

Repeating the same questions in each round allows researchers to look at how society as a whole has changed over time. But because the questions are asked of a new sample every time, these studies can only reveal change at an aggregate level – they can shed little light on who has changed, or how or why.

For example, the 2015 British Social Attitudes (PDF) survey found that 66 per cent of people thought that “it’s everybody’s duty to vote” in a general election, down from 76 per cent in 1987.

What do these findings tell us? The data clearly show us that, overall, fewer people now than in the late-1980s think that citizens have a duty to vote. We can look at the characteristics of those who do or don’t agree with this view, and how the profile of these groups had changed over time. We can also examine how the likelihood of thinking that voting is a duty has changed among different population groups (for example, different age groups or ethnicities).

These sorts of calculations would provide some very helpful insights. But there are many things that this kind of cross-sectional data cannot tell us, but which longitudinal data would help us to address. For example:

• Which individuals changed their views about voting over the period? What are their characteristics? Do some people switch, and then switch back again?
• The British Social Attitudes findings show us the net or aggregate change over time – the difference between the proportion who thought voting was a duty in 1987 and the equivalent proportion now. But this tells us nothing about change at an individual level. For example, what proportion became more inclined to think voting is a duty and what proportion became less inclined? It is important to bear in mind that if these two proportions are similar, they could cancel one another out at a net level and make it appear that very little has changed over all, even though substantial numbers of people have changed their views.
• What factors best explain the transition away from thinking voting is a duty?

Sometimes data from longitudinal studies is analysed cross-sectionally. This means that the researcher is just focusing on the information collected at one round of the study, and not linking that information to data from earlier or later rounds.

By Julia Simkus, published Dec 27, 2021

A longitudinal study is a type of observational and correlational study that involves monitoring a population over an extended period of time.

In longitudinal studies, researchers do not manipulate any variables or interfere with the environment. Instead, they simply conduct observations on the same group of subjects over a period of time.

These research studies can last as short as a week or as long as multiple years, or even decades. Unlike cross-sectional studies that measure a moment in time, longitudinal studies last beyond a single moment, enabling researchers to discover cause and effect relationships between variables.

They are beneficial for recognizing any changes, developments, or patterns in the characteristics of a target population. Longitudinal studies are often used in clinical and developmental psychology to study shifts in behaviors, thoughts, and emotions as well as trends throughout a lifetime.

For example, a longitudinal study could be used to examine the progress and well-being of children at critical age periods from birth to adulthood.

The Harvard Study of Adult Development is one of the longest longitudinal studies to date. Researchers in this study have been following the same group of men for over 80 years, observing psychosocial variables and biological processes for healthy aging and well-being in late life (see Harvard Second Generation Study).

Types of Longitudinal Studies

• A panel study is a type of longitudinal study involves sampling a cross-section of individuals at specific intervals for an extended period.
• These studies measure people’s behaviors over time, specifically their opinions, feelings, emotions and thoughts. Panel studies are a type of prospective study.

Cohort Study

• A cohort study is a type of longitudinal study that samples a group of people who share a common characteristic.
• Researchers observe a population based on the shared experience of a specific event such as birth, geographic location, or historical experience. These studies are typically used among medical researchers.

Retrospective Study

• In a retrospective study, researchers either collect data on events that have already occurred or use existing data that already exists in databases, medical records, or interviews to gain insights about a population.

Advantages

Allows researchers to look at changes overtime

Because longitudinal studies observe variables over extended periods of time, researchers can use their data to study developmental shifts and understand how certain things change as we age.

High validation

Since objectives and rules for long-term studies are established before data collection, these studies are authentic and have high levels of validity.

Eliminates recall bias

Recall bias occurs when participants do not remember past events accurately or omit details from previous experiences.

Flexibility

The variables in longitudinal studies can change throughout the study. Even if the study was created to study a specific pattern or characteristic, the data collection could show new data points or relationships that are unique and worth investigating further.

Limitations

Costly and time consuming

Longitudinal studies can take months or years to complete, rendering them expensive and time consuming. Because of this, researchers tend to have difficulty recruiting participants, leading to smaller sample sizes.

Large sample size needed

Longitudinal studies tend to be challenging to conduct because large samples are needed for any relationships or patterns to be meaningful. Researchers are unable to generate results if there is not enough data.

Participants tend to drop out

Not only is it a struggle to recruit participants, but subjects also tend to leave or drop out of the study due to a variety of reasons such as illness, relocation, or a lack of motivation to complete the full study. This tendency is known as selective attrition and can threaten the validity of an experiment. For this reason, researchers using this approach typically recruit many participants fully expecting that a substantial number will drop out before the end.

Report bias is possible

Longitudinal studies will sometimes rely on surveys and questionnaires which could result in inaccurate reporting as there is no way to verify the information presented.   

Examples

• LeMare and Audet (2006) carried out a longitudinal study on the physical growth and health of 36 Romanian orphans adopted by Canadian families and compared them to a group of children raised in normal Canadian families.

  Data were collected for each child at three time points: at 11 months after adoption, at 4.5 years of age and at 10.5 years of age.

  The first two sets of results showed that the adoptees were behind the non-institutionalised group however by 10.5 years old there was no difference between the two groups. The Romanian orphans had caught up with the children raised in normal Canadian families.

• The role of positive psychology constructs in predicting mental health and academic achievement in children and adolescents (Marques Pais-Ribeiro, & Lopez, 2011)
• The correlation between dieting behavior and the development of bulimia nervosa (Stice et al., 1998)
• The stress of educational bottlenecks negatively impacting students’ wellbeing (Cruwys, Greenaway, & Haslam, 2015)
• The effects of job insecurity on psychological health and withdrawal (Sidney & Schaufeli, 1995)
• The relationship between loneliness, health, and mortality in adults aged 50 years and over (Luo et al., 2012)
• The influence of parental attachment and parental control on early onset of alcohol consumption in adolescence (Van der Vorst et al., 2006)
• The relationship between religion and health outcomes in medical rehabilitation patients (Fitchett et al., 1999)

How to Perform a Longitudinal Study

When beginning to develop your longitudinal study, you have to first decide if you want to collect your own data or use data that has already been gathered.

Using already collected data will save you time, but the data will be more restricted and limited than if you collect it yourself. When collecting your own data, you can choose to conduct either a retrospective or prospective study.

In a retrospective study, you are collecting data on events that have already occurred. You can examine historical information, such as medical records, in order to understand the past. In a prospective study, on the other hand, you are collecting data in real time.

Prospective studies are more common for psychology research. Once you determine the type of longitudinal study you will conduct, you then must determine how, when, where, and on whom the data will be collected.

A standardized study design is vital for efficiently measuring a population. Once a study design is created, it is important that researchers maintain the same study procedures over time to uphold the validity of the observation.

A schedule should be maintained, complete results should be recorded with each observation, and observer variability should be minimized.

Researchers must observe each subject under the same conditions in order to compare them. In this type of study design, each subject is the control.

Longitudinal vs Cross-Sectional Studies

Longitudinal studies and cross-sectional studies are two different observational study designs where researchers are analyzing a target population without manipulating or altering the natural environment in which the participants exist.

Yet, there are apparent differences between these two forms of study. One key difference is that longitudinal studies follow the same sample of people over an extended period of time while cross-sectional studies look at the characteristics of different populations at a given moment in time.

Longitudinal studies tend to require more time and resources, but they can be used to detect cause-and-effect relationships and establish patterns among subjects.

Cross-sectional studies, on the other hand, tend to be cheaper and quicker but are only able to provide a snapshot of a point in time and thus cannot identify cause-and-effect relationships.

Both types of studies are valuable for psychologists to observe a given group of subjects, but cross-sectional studies are more beneficial for establishing associations between variables while longitudinal studies are necessary for examining a sequence of events.

Frequently asked questions about longitudinal studies

1. Are longitudinal studies qualitative or quantitative?

Both! Like cross-sectional studies, longitudinal studies can be either quantitative or qualitative.

2. What's the difference between a longitudinal and case-control study?

Case control studies compare groups retrospectively and cannot be used to calculate relative risk. Longitudinal studies, though, can compare groups either retrospectively or prospectively. In case-control studies, researchers study one group of people who have developed a particular condition and compare them to a sample without the disease. Case-control studies look at a single subject or a single case, whereas longitudinal studies are conducted on a large group of subjects.

3. Does a longitudinal study have a control group?

Essentially, yes. Since researchers observe each subject under the same conditions, each subject is an individual control.

Julia Simkus is an undergraduate student at Princeton University, majoring in Psychology. She plans to pursue a PhD in Clinical Psychology upon graduation from Princeton in 2023. Julia has co-authored two journal articles, one titled “Substance Use Disorders and Behavioral Addictions During the COVID-19 Pandemic and COVID-19-Related Restrictions," which was published in Frontiers in Psychiatry in April 2021 and the other titled “Food Addiction: Latest Insights on the Clinical Implications," to be published in Handbook of Substance Misuse and Addictions: From Biology to Public Health in early 2022.

How to reference this article:

Simkus, J. (2021, Dec 27). What is a Longitudinal Study? Simply Psychology. www.simplypsychology.org/longitudinal-study.html

Sources

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Fitchett, G., Rybarczyk, B., Demarco, G., & Nicholas, J.J. (1999). The role of religion in medical rehabilitation outcomes: A longitudinal study. Rehabilitation Psychology, 44, 333-353.

Harvard Second Generation Study. (n.d.). Harvard Second Generation Grant and Glueck Study. Harvard Study of Adult Development. Retrieved from https://www.adultdevelopmentstudy.org.

Le Mare, L., & Audet, K. (2006). A longitudinal study of the physical growth and health of postinstitutionalized Romanian adoptees. Paediatrics & child health, 11(2), 85-91.

Luo, Y., Hawkley, L. C., Waite, L. J., & Cacioppo, J. T. (2012). Loneliness, health, and mortality in old age: a national longitudinal study. Social science & medicine (1982), 74(6), 907–914. https://doi.org/10.1016/j.socscimed.2011.11.028

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Tegan Cruwys, Katharine H Greenaway & S Alexander Haslam (2015) The Stress of Passing Through an Educational Bottleneck: A Longitudinal Study of Psychology Honours Students,Australian Psychologist, 50:5, 372-381, DOI: 10.1111/ap.12115

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