past. The selection of individuals into the study—unlike that for cohort and case-control studies—is independent of both the exposure to the agent under study and disease characteristics. Cross-sectional studies seek to uncover potential associations between exposure to specific agents and development of disease. They may compare disease or symptom rates between groups with and without the exposure to the specific agent or may compare exposure to the specific agent between groups with and without the disease. Although cross-sectional studies need not have control groups, studies with control groups are more methodologically sound. Several health studies of Gulf War veterans are controlled cross-sectional surveys that compare a sample of veterans previously deployed to the Gulf War with a sample of veterans who served during the same period but were not deployed to the Gulf War (see Chapter 2).
Cross-sectional surveys are easy to perform and inexpensive to implement relative to cohort studies. Cross-sectional surveys can identify the prevalence of diseases and exposures in a defined population. They are useful for generating hypotheses; however, they are much less useful for determining cause-and-effect relationships, because disease and exposure data are collected simultaneously and may be self-reported (Monson, 1990). It may also be difficult to determine the temporal sequence of exposure and symptoms or disease.
Experimental studies in humans are the foremost means of establishing causal associations between exposures and human health outcomes. Experimental studies are used most frequently in the evaluation of the safety and efficacy of medications, surgical practices, biological products, vaccines, and preventive interventions. In an experiment, the investigator has control over assigning the agent to be studied and recording the outcome. Two key features of experimental studies are prospective design and use of a control group. Randomized controlled trials are considered the gold standard in experimental studies.
In randomized controlled trials, each subject has a known, often equal, probability of assignment to either the treatment or the control group. Large randomized controlled trials are designed to have all possible confounding variables occur with equal frequency in the intervention and control groups. Blinding may be another aspect of randomized controlled trials.8 Blinding refers to shielding subjects or investigators from knowledge of whether the subjects were assigned to the treatment or the control group. Blinding is most readily accomplished when subjects in the control group receive a placebo. When both subjects and investigators are unaware of patient assignment, the study is said to be “double-blind.” The objective of blinding is to reduce bias introduced by patients’ and
Blinding can also be part of the study design in cohort and case control studies. Disease outcome and exposures can be determined independently by different groups of researchers or the exposure assessment in case control studies can be performed by scientists who are blinded as to the disease status of the subjects.