What is a Longitudinal Study?
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.
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.
- 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.
- 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.
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.
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.
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.
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.
- 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.
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.