Longitudinal studies: what they are and how they work in research

Longitudinal studies are a research method that consists of measuring a phenomenon through a certain time interval. In this sense, they serve to analyze and observe in a sequenced manner the evolution of a phenomenon or its elements. They are frequently used in research related to health sciences, as well as statistics, psychology, sociology, and education.

In this article we will see that it is a longitudinal study, and what are some of its main features and uses.

• Related article: "The 15 types of research (and characteristics)"

What is a longitudinal study and what is it for?

Longitudinal studies are a research method, which as such, is a structured set of procedures that allows us to obtain information on a given topic.

Specifically, the goal of longitudinal studies is to obtain information about a change process. They also serve to estimate incidents and anticipate risks. This can occur through the observation and measurement of individual patterns and their stability or modifications over time.

In other words, they allow estimates of the rate of change as a function of time, and in relationship with different characteristics of the individual such as age or other conditions (Arnau and Bono, 2008).

In this sense, longitudinal studies have traditionally been considered research methods. opposed to cross-sectional studies, which are based on momentary or fixed observations at a given point in time, although they are not unrelated to each other.

• You may be interested in: "What is the scientific method and how does it work?"

Disciplines that use it and related studies

Longitudinal studies are especially used in health sciences, but they also allow measure the evolution of some phenomena also in psychology, education, sociology or demography, to name a few.

In turn, the term "longitudinal study" may have some variations according to the specific discipline that uses it. For example, if these are studies carried out in the field of sociology, longitudinal studies are related to a type of study called "panel study"; while when it comes to studies in the epidemiological and demographic field it is a subtype of the classic cohort study (those that measure the elements of a phenomenon between two or more intervals temporary).

In relation to the above, another type of cohort study is life tables. The difference between a life table and a longitudinal study is that the first makes a measurement considering only the beginning and the end of the interval (that is, the phenomenon is observed twice, once at the beginning and once at the end, and the data is analyzed from there). In contrast, in the longitudinal study measurements are made repeatedly (Delgado, M. and Llorca, J., 2004).

Likewise, when it comes to studies that are applied in the statistical field, they have also been considered as a type of repeated measurements study. They are called this way because they are a type of study based on repetitive measurement, that is, they allow observe a certain number of appearances of the phenomenon or of some of its characteristics in a given time specific.

Types of longitudinal research

Depending on the specific field in which the longitudinal study is applied, it can be of different types. To illustrate, we will briefly describe its characteristics in epidemiology and statistics.

1. in epidemiology

The basis of the longitudinal study used in epidemiology is to know the disease experience of a population over time. Allow know the transitions between states of health and disease, and incorporate variables such as age or gender.

2. in statistics

This is a study consisting of make more than two measurements over a period of time. That is to say, it is not just a question of measuring a phenomenon at the beginning and another at the end, but of making repeated measurements of the phenomenon. In turn, this can be applied in different fields, for example in developmental psychology.

Design of this form of research

As with all research methods, longitudinal studies are applied according to the specific objective that the research pursues. The description of the studies and the elements that make it up and that will allow it to be carried out, is what we know as research design.

The design of a study is important as it allows to ensure that the methodology will correspond to the objectives and will make it possible to reach results consistent with them. In this case, longitudinal studies are used in research that aims to discover a process of change over time.

Although the specific design depends on the type of longitudinal study to be conducted, as well as the field application-specific, broadly speaking this type of research requires including the following items:

• Longitudinal data, which is the number of repetitions in which the phenomenon will be observed.
• Observed elements can be units, individuals, subjects, groups, populations.
• Time points, which are the time intervals at which the item is recorded, can range from a few minutes to several years.
• Response Profile, also called trend or curve, which is the response set of the measured unit.

limitations

Both in longitudinal studies and in other studies based on repeated measures, there are two fundamental implications. The first is that there is a dependency between the number of times the phenomenon is repeated and the observed unit. In other words, the number of repetitions is the main criterion for explaining the phenomenon under study.

The second is that the circumstances or variable under which the phenomenon can be repeated, frequently out of the control of the person investigating, with which, the data can often be incomplete.

Bibliographic references:

• Arnau, J. and Bono, R. (2008). Longitudinal studies of repeated measures. Design models and analysis. Psychology Writings, 2(1): 32-41.
• Delgado, m. and Llorca, J. (2004). Longitudinal studies: concept and particularities. Spanish Journal of Public Health, 78: 141-148.