Epidemiologists often perform case-control studies because they are relatively easy to conduct. In these studies, cases are sampled according to a predetermined protocol, and the controls selected must be comparable with the cases.50,5152 However, identifying a suitable control population is often difficult for two reasons: (1) identifying the characteristics that ensure comparability with the cases is often difficult, and (2) to avoid selection biases, one must sample the participants from the control population according to a well-controlled scheme. To ensure random sampling, researchers must select controls at random. Often, investigators match controls with the cases according to criteria that are known to be associated with disease risk. Matching cases and controls minimizes the variability among the subjects according to known risk factors that may not be of interest. For instance, controls may be grouped with cases according to demographic characteristics such as age, ethnicity, and sex. In such instances, researchers usually perform a conditional analysis, in which each case is matched with a control or set of controls.
The control population should be comparable with the case population in terms of socioeconomic status and other unmeasured factors. Several methods of accruing controls can be used. One is random-digit telephone dialing. However, one should note that one fourth of all residential telephone subscribers use caller identification devices and thus are able to screen out unfamiliar callers. As a result, using this method would increase the potential for sample bias. Another popular method for selecting controls in the United States has been the use of data tapes obtained from motor vehicle bureaus, but because older adults may no longer have driver's licenses, this method may exclude potential controls. Furthermore, several states have passed legislation banning distribution of lists of registered drivers containing their names and addresses.
One may have to make a tradeoff when performing case-control epidemiological studies. Although population-based studies are inarguably the gold standard for modern epidemiological case-control research, one may be unable to conduct population-based studies for practical reasons. For example, population-based research may not be feasible when studying very rare pain conditions (e.g. neuropathic pain) for which the catchment population is prohibitively large or when studying pain conditions associated with very lethal diseases (e.g. cancer-related pain) because the population-based ascertainment may fail to provide a representative sample of cases as a result of patient deaths prior to enrollment.
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