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Presented at a Webinar on Research Methodology in Health Sciences at Northern Area Armed Forces Hospital (NAAFH) on 5th September 2022. By Professor Omar Hasan Kasule Sr. MB ChB (MUK). MPH (Harvard), DrPH (Harvard) Professor of Epidemiology and Bioethics
LEARNING OBJECTIVES:
- Follow-up studies: Definition and types.
- Follow-up studies: Design an analysis.
- Follow-up studies: Strengths and weaknesses.
KEYWORDS and TERMS:
- Cohort study: closed cohort, open cohort.
- Follow-up study: ambispective, retrospective, prospective.
- Loss to follow-up.
DEFINITION:
- A follow-up study (also called cohort study, incident study, prospective study, or longitudinal study), compares disease in exposed to disease in non-exposed groups after a period of follow-up.
- A follow-up study can be prospective (forward), retrospective (backward), or ambispective (both forward and backward) follow-up.
- The follow-up cohorts may be closed (fixed cohort) or open (dynamic cohort).
- Analysis of fixed cohorts is based on CI and that of open cohorts on IR.
STUDY DESIGN:
- The study population is divided into the exposed and unexposed populations.
- A sample is taken from the exposed and another sample is taken from the unexposed.
- Both the exposed and unexposed samples are followed for the appearance of disease.
SOURCES of COHORTS:
- Special exposure groups such as factory workers.
- Groups offering special resources such as health insurance subscribers.
- Institutionalized such as the army, and police.
SOURCES of EXPOSURE INFORMATION:
- Existing records,
- Interviews/questionnaires,
- Medical examinations,
- Laboratory tests for biomarkers,
- Testing or evaluation of the environment.
OUTCOME ASSESSMENT:
- The time of occurrence of the outcome must be defined precisely.
- The ascertainment of the outcome event must be standardized with clear criteria.
- Follow-up can be achieved by letter, telephone, surveillance of death certificates, and hospitals.
- Care must be taken to make sure that surveillance, follow-up, and ascertainment for the 2 groups are the same.
PROBLEMS of FOLLOW-UP STUDIES:
- Non-response.
- Loss to follow up.
- Bias: selection bias, follow-up bias, information/misclassification bias, confounding bias.
STATISTICAL PARAMETERS:
- Both incidence and risk statistics can be computed.
- The incidence statistics are the incidence rate and the cumulative incidence.
- The risk statistics are either the risk difference or the various ratio statistics (risk ratio, the rate ratio, the relative risk, or the odds ratio).
ADVANTAGES of the FOLLOW-UP DESIGN:
- True risk ratio based on incidence rates,
- The time sequence is clear since exposure precedes disease,
- Incidence rates can be determined directly,
- Several outcomes of the same exposure can be studied simultaneously.
DISADVANTAGES of the FOLLOW-UP
DESIGN:
- Loss of subjects and interest due to long follow-up,
- Inability to compute the prevalence rate of the risk factor,
- Use of large samples to ensure enough cases of the outcome,
- High cost,
- Follow-up studies are not suitable for the study of diseases with low incidence.
2 x2 TABLE:
