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Presented at CRC course KFMC on January 26, 2020 11-12am by Prof Omar Hasan Kasule Sr Professor of Epidemiology and Bioethics, King Fahad Medical City.
Learning Objectives:
• Follow-up studies: definition and types
• Follow-up studies: design and analysis
• Follow-up studies: strengths and weaknesses
• Determination of the sample size
Key Words and Terms:
• Cohort: study, closed cohort, open cohort
• Cumulative incidence
• Follow-up bias, study, ambispective, retrospective, prospective
• Loss to follow-up
DEFINITION:
• A follow-up study (also called a cohort study, incident study, prospective study, or longitudinal study), compares disease in exposed to the 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.
• In a nested case-control design, a case-control study is carried out within a larger follow-up study.
• 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 the disease.
• The study may include matching, (one-to-one or one-to-many), pre and post comparisons, multiple control groups, and stratification.
SOURCES OF COHORTS:
• special exposure groups such as factory workers
• Groups offering special resources such as health insurance subscribers.
• Institutionalized such as army, 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 NON RESPONSE IN FOLLOW UP STUDIES:
• In non-random non-response on exposure, the risk ratio is valid but the distribution of exposure in the community is not valid.
• In non-random non-response on the outcome, the odds ratio is valid but the disease incidence rate is not valid.
• There is a more complex situation when there is non-response on both exposure and outcome.
• In general random non-response is better than non-random or differential non-response.
PROBLEM OF LOSS TO FOLLOW UP:
• Loss to follow-up can be related to the outcome, the exposure, and to both outcome and exposure.
• The consequences of loss to follow-up are similar to those of non-response.
• In cases of regular follow-up, it is assumed that the loss occurred immediately after the last follow-up.
• If the loss to follow-up is related to an event such as death, it can be assumed that the loss was halfway between the last observation and the death.
FIVE TYPES OF BIAS CAN ARISE IN FOLLOW-UP STUDIES:
• Selection bias arises when the sample is not representative of the population.
• Follow-up bias arises when the loss to follow-up is unequal among the exposed and the unexposed, when disease occurrence leads to loss to follow-up, when people may move out of the study area because of the exposure being studied, and when the observation of the two groups is unequal.
• Information/misclassification bias arises due to measurement error or misdiagnosis.
• Confounding bias arises usually due to age and smoking because both are associated with many diseases.
• Post-hoc bias arises when cohort data is used to make observations that were not anticipated before.
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,
• 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 prevalence rate of the risk factor,
• Use of large samples to ensure enough cases of the outcome,
• High cost: cost can be decreased by using existing monitoring/surveillance systems, historical cohorts, general population information instead of studying the unexposed population, and the nested case-control design.
• Follow-up studies are not suitable for the study of diseases with low incidence.