CONCEPTS UNDERLYING STUDY DESIGN Nigel Paneth

DESCRIPTIVE VS ANALYTICAL STUDIES Descriptive studies often focus on a single variable such as a particular disease or health state. They frequently have no etiologic hypothesis, restricting their interest to disease frequency. A common question answered by a descriptive study in epidemiology might be: How common is this disease in this place?

THE MAIN DESCRIPTIVE STUDY IN EPIDEMIOLOGY: THE COMMUNITY SURVEY A community survey is a study which attempts to ascertain the frequency of a disease in a fixed geographic region (ideally) or in a group defined by a common membership, e.g. school-children. It usually assesses the frequency of disease in easily ascertained sub-groups, i.e. age, gender, geographic sub-units, ethnicity. etc. It is often the first step in looking at risk factors for disease.

ANALYTIC STUDIES Unlike descriptive studies, analytic studies in epidemiolgy must have a clear hypothesis (usually an etiologic hypothesis). Thus at least two variables must be specified in advance, an exposure and a disease or health state. A hypothesis might be “What is the association between this exposure and this disease?” But quantitative prediction is the key to good hypothesis formulation. So ideally we specify the size of the association – for example “I predict that this exposure will be have a RR of at least 2.0 with the incidence of this disease.”

OBSERVATIONAL, EXPERIMENTAL, AND QUASI-EXPERIMENTAL STUDIES

OBSERVATIONAL STUDY In an observational study, the exposure is not assigned by the scientist doing the study. For example, a study in which cigarette smokers are compared to non-smokers for lung cancer incidence.

EXPERIMENT In epidemiologic research (not necessarily in all science), an experiment is a study in which an exposure is assigned randomly

QUASI-EXPERIMENT In epidemiologic research, a quasi-experiment is a study in which an intervention is assigned by the investigator, but not randomly. For example, a new educational program is started in a classroom, and the results are compared to those found in a traditional classroom, composed of similar students.

ECOLOGICAL VS INDIVIDUAL STUDIES

ECOLOGICAL STUDIES In experimental studies, a study is defined as ecological, when the assignment is to a group – e.g. assigning villages, not individuals, to the different arms of the study. In observational studies, the study is ecological when the unit of outcome analysis is a group – e.g. the distribution of Lyme disease by county is compared to the distribution of tick species by county.

INDIVIDUAL STUDIES In individual studies, the unit of outcome in the analysis is an individual. An individual study, can however, have an ecological exposure (which is why observational ecological studies are defined as ecological by outcome. For example, an individual study of lung cancer may use an individual exposure (smoking by participant) or an ecological exposure (urban or rural residence)

THE ECOLOGICAL FALLACY 1. Statistical meaning: The tendency of correlation coefficients to be larger when an association is assessed at the group level than when it is assessed at the individual level   2. Common usage: The tendency of ecological studies to imprecisely reflect reality as documented in individual studies.

EXAMPLE OF ECOLOGICAL FALLACY Countries with high salt intakes have higher levels of hypertension. But it has been hard to show that salt intake and hypertension are related in individuals. It may be that within countries, individuals are above or below some threshold of the relationship of salt intake to hypertension.

ATOMISTIC FALLACY This fallacy is the opposite of the ecological fallacy, and refers to the assumption that associations found at the individual level will necessarily be replicated at the group level.  For example, the major determinant of infant mortality is the birth weight of the baby. But it is not necessarily true that the differences between countries in infant mortality are due to differences in their birth weight distributions. (although in fact they often are)

KRAMER AND BOIVIN'S THREE DIMENSIONS OF RESEARCH DESIGN 1. DIRECTIONALITY-The direction in which exposure and outcome are investigated – from exposure to outcome, from outcome to exposure, or both simultaneously  2. SAMPLE SELECTION-Criteria used to choose study subjects; based on exposure, outcome or other criteria.  3. TIMING-Relation between the time of the study and the calendar times of exposure and outcome; historical, concurrent, mixed.

DIRECTIONALITY is the key determinant of whether a study is cohort or case-control. A COHORT STUDY IS ONE IN WHICH SUBJECTS ARE INVESTIGATED FORWARD FROM EXPOSURE TO OUTCOME. A CASE-CONTROL STUDY IS ONE IN WHICH SUBJECTS ARE INVESTIGATED BACKWARDS FROM OUTCOME TO EXPOSURE.

SAMPLE SELECTION The sampling procedure is not the determinant of whether a study design is cohort or case-control, since one is not required to sample by exposure in cohort studies, or sample by outcome in case-control studies. Thus the way the sample is chosen does not determine the type of study.

SAMPLING AND STUDY DESIGN Sampling follows directionality in case-control studies, in that sampling of the outcome is the usual starting point. In cohort studies, however, the commonest strategy is sampling a population (e.g. Framingham, etc.) not sampling by exposure, although that is sometimes seen (exposure-control study).

TIMING Timing also does not determine whether a study is cohort or case-control. Cohort studies can be retrospective (historical) or prospective in relation to calendar time. Moreover, even in a prospective cohort study, baseline exposure information that is historical is commonly collected.

Case-control studies are usually viewed as retrospective, although Kramer and Boivin view them as mixed, because the outcome status can be ascertained in the present (but doesn’t have to be) Cross-sectional studies are almost always done in the present, but could be done in the past (i.e. through reviewing old records)

CONCEPT OF STUDY BASE The study base is the idealized source population from which the actual study population is selected. It is described in terms of the person-year experience of that source population in relation to exposure and disease. In some study designs, the study base is obvious, in others, it is not so clear. It is always useful to try to imagine the study base of each study. Deviations of the actual study population composition from the study base can help explain problems with the study findings.  We will return to this concept in more detail when we study case control studies.