Longitudinal studies employ continuous or repeated measures to follow particular individuals over prolonged periods of time—often years or decades. They are generally observational in nature, with quantitative and/or qualitative data being collected on any combination of exposures and outcomes, without any external influenced being applied. This study type is particularly useful for evaluating the relationship between risk factors and the development of disease, and the outcomes of treatments over different lengths of time. Similarly, because data is collected for given individuals within a predefined group, appropriate statistical testing may be employed to analyze change over time for the group as a whole, or for particular individuals.
In contrast, cross-sectional analysis is another study type that may analyze multiple variables at a given instance, but provides no information with regards to the influence of time on the variables measured—being static by its very nature. It is thus generally less valid for examining cause-and-effect relationships. Nonetheless, cross-sectional studies require less time to be set up, and may be considered for preliminary evaluations of association prior to embarking on cumbersome longitudinal-type studies.
Longitudinal study designs
Longitudinal research may take numerous different forms. They are generally observational, however, may also be experimental. Some of these are briefly discussed below:
(i) Repeated cross-sectional studies where study participants are largely or entirely different on each sampling occasion;
(ii) Prospective studies where the same participants are followed over a period of time. These may include:
- Cohort panels wherein some or all individuals in a defined population with similar exposures or outcomes are considered over time;
- Representative panels where data is regularly collected for a random sample of a population;
- Linked panels wherein data collected for other purposes is tapped and linked to form individual-specific datasets.
(iii) Retrospective studies are designed after at least some participants have already experienced events that are of relevance; with data for potential exposures in the identified cohort being collected and examined retrospectively.
Advantages of Longitudinal Study
Longitudinal cohort studies, particularly when conducted prospectively in their pure form, offer numerous benefits. These include:
(i) The ability to identify and relate events to particular exposures, and to further define these exposures with regards to presence, timing and chronicity;
(ii) Establishing sequence of events;
(iii) Following change over time in particular individuals within the cohort;
(iv) Excluding recall bias in participants, by collecting data prospectively and prior to knowledge of a possible subsequent event occurring, and;
(v) Ability to correct for the “cohort effect”—that is allowing for analysis of the individual time components of cohort (range of birth dates), period (current time), and age (at point of measurement)—and to account for the impact of each individually.
Disadvantages of Longitudinal Study
Numerous challenges are implicit in the study design; particularly by virtue of this occurring over protracted time periods. We briefly consider the below:
(i) Incomplete and interrupted follow-up of individuals, and attrition with loss to follow-up over time; with notable threats to the representative nature of the dynamic sample if potentially resulting from a particular exposure or occurrence that is of relevance;
(ii) Difficulty in separation of the reciprocal impact of exposure and outcome, in view of the potentiation of one by the other; and particularly wherein the induction period between exposure and occurrence is prolonged;
(iii) The potential for inaccuracy in conclusion if adopting statistical techniques that fail to account for the intra-individual correlation of measures, and;
(iv) Generally-increased temporal and financial demands associated with this approach.
Longitudinal methods may provide a more comprehensive approach to research, that allows an understanding of the degree and direction of change over time. One should carefully consider the cost and time implications of embarking on such a project, whilst ensuring complete and proven clarity in design and process, particularly in view of the protracted nature of such an Endeavour; and noting the peculiarities for consideration at the interpretation stage.