Cohort studies description

The types of epidemiologic studies used by risk assessors include case-control studies, cohort studies, descriptive epidemiologic studies, and case reports ... 

Descriptive as well as case-control and cohort studies have shown that tamoxifen may cause alterations, including cancer, in human endometrium. The findings have been observed in studies using ultrasound technology or histology data. 

Occupational exposure to silica dust has been identified as a risk factor for several systemic autoimmune diseases. This literature dates back almost 100 years, to the description by Bramwell of diffuse scleroderma in stone masons.26 Rheumatoid arthritis and scleroderma in miners were described in the 1950s, and more formal cohort studies of miners and of granite workers were conducted in the 1980s. Other studies focusing on silicosis patients, and several case-control studies of these diseases and of Wegner... 

The following three accounts of waste-site investigations demonstrate four major approaches to assessing health effects at hazardous waste sites (1) Descriptive studies, (2) case-control studies, (3) studies, and (4) cohort studies. 

The major advantage of cohort studies is that exposure is measured before disease occurs and thus provides strong evidence of causality, given that the exposure of interest will be unlikely to be affected by disease status. Other advantages of cohort studies are that they allow measurement of disease in the exposed and unexposed population, can measure multiple outcomes, can evaluate rare exposures, and are not subjected to some types of biases, such as recall bias (see Section 26.2.4 for a description of biases). Disadvantages include (a) the requirement of large number of subjects, (b) expense, (c) requirement of an extensive time to set up and follow up, (d) cannot be used to evaluate rare diseases, and (e) can be associated with some type of biases such as selection bias. 

Advantages of case-control studies are that they are cheaper and less time-consuming than cohort studies, they allow for the evaluation of multiple exposures, and they are good for the study of rare diseases. Disadvantages are that they do not provide information on incidence of disease, they are not good for evaluating rare exposures, they assess exposure after disease has occurred, and they are subjected to many types of biases, such as selection and recall (see Section 26.2.4 for a description of biases). 

The North Carolina Breast Milk and Formula Project. The North Carolina Breast Milk and Formula Project (NCBMFP) is a cohort study designed to assess the relationship between exposure to prenatal and postnatal PCBs and DDE and growth and development in infants and children (Rogan et al. 1986a, 1986b). A detailed description of this cohort study in presented in Section 3.2.4.2.1.2 (Neurological Effects). Briefly, the participants were administered a questionnaire while in the hospital following delivery. Maternal serum, cord blood, and placenta samples were collected at birth as well as colostrum, breast milk, or formula. The first follow-up visit occurred at 6 weeks with subsequent evaluations at... 

Descriptive epidemiologic studies do not have a temporal component like case-control or cohort studies. Rather, this type of study evaluates factors that may influence the incidence of a disease, such as demographic or socioeconomic characteristics. It is not possible to determine causality from a descriptive epidemiologic study. Rather, this type of study is often used to generate a hypothesis that can be tested in case-control or cohort studies. 

Like case-control studies, prospective cohort studies are less subject to the biases of descriptive studies, because the exposure data, in this case vitamin C... 

Quality of evidence I, evidence from >1 properly randomized, controlled trial II, evidence from <1 well-designed clinical trial, without randomization from cohort or case-controlled analytic studies (preferably from >1 center) or from multiple time-series III, evidence from opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees. 

The number of subjects per cohort needed for the initial study depends on several factors. If a well established pharmacodynamic measurement is to be used as an endpoint, it should be possible to calculate the number required to demonstrate significant differences from placebo by means of a power calculation based on variances in a previous study using this technique. However, analysis of the study is often limited to descriptive statistics such as mean and standard deviation, or even just recording the number of reports of a particular symptom, so that a formal power calculation is often inappropriate. There must be a balance between the minimum number on which it is reasonable to base decisions about dose escalation and the number of individuals it is reasonable to expose to a NME for the first time. To take the extremes, it is unwise to make decisions about tolerability and pharmacokinetics based on data from one or two subjects, although there are advocates of such a minimalist approach. Conversely, it is not justifiable to administer a single dose level to, say, 50 subjects at this early stage of ED. There is no simple answer to this, but in general the number lies between 6 and 20 subjects. 

Polybrominated Biphenyls. Hypothyroidism was diagnosed in 4 of 35 men who were occupationally exposed to unspecified PBBs and/or decaBDE (Bahn et al. 1980). The cohort consisted of workers (mean age 35.9 years) who had been employed at a production plant for at least 6 weeks during a 52-month period during which PBBs and decaBDE were the only chemicals manufactured and who had volunteered for a comprehensive medical evaluation performed 3 months after the end of the 52-month period. There was no further description of exposure, and it was assumed to have occurred by both inhalation and dermal routes. As detailed in Section 3.2.1.2, the results of this study suggest that occupational exposure to PBBs, decaBDE, and/or bromine affected the thyroid, but the mixed chemical exposure and a lack of data on serum or tissue levels of the chemicals preclude attributing effects solely to any particular congener or mixture of congeners. 

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