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Background material for Year 3 Semester 1 Med PPSD session on 3rd November 2008
1.0 CROSS-SECTIONAL DESIGN
1.1 DEFINITION
The cross-sectional study, also called the prevalence study or naturalistic sampling, has the objective of determination of prevalence of risk factors and prevalence of disease at a point in time (calendar time or an event like birth or death). Disease and exposure are ascertained simultaneously. A cross-sectional study can be descriptive or analytic or both. It may be done once or may be repeated. Individual-based studies collect information on individuals. Group-based (ecologic) studies collect aggregate information about groups of individuals. Cross-sectional studies are used in community diagnosis, preliminary study of disease etiology, assessment of health status, disease surveillance, public health planning, and program evaluation. Cross-sectional studies have the advantages of simplicity, and rapid execution to provide rapid answers. Their disadvantages are: inability to study etiology because the time sequence between exposure and outcome is unknown, inability to study diseases with low prevalence, high respondent bias, poor documentation of confounding factors, and over-representation of diseases of long duration.
1.2 ECOLOGIC DESIGN
Ecological studies, exploratory or analytic, study aggregate and not individual information. Groups commonly used are schools, factories, and countries. Exposure is measured as an overall group index. Outcome is measured as rates, proportions, and means. The correlation and regression coefficients are used as effect measures. The advantages of ecological studies are: low cost, convenience, easy analysis, and interpretation. They have several weaknesses. They generate but cannot test hypotheses. They cannot be used in definitive etiological research. They suffer from ecological fallacy (relation at the aggregate is not true at the individual level). They lack data to control for confounding. Data is often inaccurate or incomplete. Collinearity is a common problem.
1.2 HEALTH SURVEYS
Surveys involve more subjects than the usual epidemiological sample are used for measurement of health and disease, assessment of needs, assessment service utilization and care. They may be population or sample surveys. Planning of surveys includes: literature survey, stating objectives, identifying and prioritizing the problem, formulating a hypothesis, defining the population, defining the sampling frame, determining sample size and sampling method, training study personnel, considering logistics (approvals, manpower, materials and equipment., finance, transport, communication, and accommodation), preparing and pre-testing the study questionnaire. Surveys may be cross sectional or longitudinal. The household is the usual sampling unit. Sampling may be simple random sampling, systematic sampling, stratified sampling, cluster sampling, or multistage sampling. Existing data may be used or new data may be collected using a questionnaire (postal, telephone, diaries, and interview), physical examinations, direct observation, and laboratory investigations. Structure and contents of the survey report is determined by potential readers. The report is used to communicate information and also apply for funding.
2.1 BASICS
The case-control study is popular because or its low cost, rapid results, and flexibility. It uses a small numbers of subjects. It is used for disease (rare and non rare) as well as non disease situations. A case control study can be exploratory or definitive. The variants of case control studies are the case-base, the case-cohort, the case-only, and the crossover designs. In the case-base design, cases are all diseased individuals in the population and controls are a random sample of disease-free individuals in the same base population. The case-cohort design is sampling from a cohort (closed or open). The case-only design is used in genetic studies in which the control exposure distribution can be worked out theoretically. The crossover design is used for sporadic exposures. The same individual can serve as a case or as a control several times without any prejudice to the study. The source population for cases and controls must be the same. Cases are sourced from clinical records, hospital discharge records, disease registries, data from surveillance programs, employment records, and death certificates. Cases are either all cases of a disease or a sample thereof. Only incident cases (new cases) are selected. Controls must be from the same population base as the cases and must be like cases in everything except having the disease being studied. Information comparability between the case series and the control series must be assured. Hospital, community, neighborhood, friend, dead, and relative controls are used. There is little gain in efficiency beyond a 1:2 case control ratio unless control data is obtained at no cost. Confounding can be prevented or controlled by stratification and matching. Exposure information is obtained from interviews, hospital records, pharmacy records, vital records, disease registry, employment records, environmental data, genetic determinants, biomarker, physical measurements, and laboratory measurements.
2.2 STRENGTHS AND WEAKNESSES
The case-control study design has the following strengths/advantages: computation of the OR as an approximation of the RR, low cost, short duration, and convenience for subjects because they are contacted/interviewed only once. The case control design several disadvantages: RR is approximated and is not measured, Pr(E+/D+) is computed instead of the more informative Pr(D+/E+), rates are not obtained because marginal totals are artificial and not natural being fixed by design, the time sequence between exposure and disease outcome is not clear, vulnerability to bias (misclassification, selection, and confounding), inability to study multiple outcomes, lack of precision in evaluating rare exposures, inability to validate historical exposure information, and inability to control for relevant confounding factors.
3.1 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. It 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). 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 appearance of disease. The study may include matching, (one-to-one or one-to-many), pre and post comparisons, multiple control groups, and stratification. The study cohort is from special exposure groups, such as factory workers, or groups offering special resources, such as health insurance subscribers. Information on exposure can be obtained from the following sources: existing records, interviews/questionnaires, medical examinations, laboratory tests for biomarkers, testing or evaluation of the environment. 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.
3.2 STRENGTHS and WEAKNESSES
The cohort design has 4 advantages: it gives a true risk ratio based on incidence rates, the time sequence is clear since exposure precedes disease, incidence rates can be determined directly, and several outcomes of the same exposure can be studied simultaneously. It has 5 disadvantages: loss to 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 outcome, and high cost. The 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 study of diseases with low incidence.