Question

Suppose a researcher wants to study environmental risk factors for cancer in New Brunswick and chooses a case-control design. He is having trouble recruiting controls for the study, and so starts to advertise in Maine newspapers to increase his numbers. Why is this a problem?

Answer 1

This is a problem because Cases can be selected from hospitals, registries, or other relevant sources. However, cases based on hospitals may be a biased sample; for
example, those cases seen at referral hospitals may represent more serious or unusual cases. Therefore, population-based case ascertainment is the preferred study design. This may be possible through a cancer registry if the registry can provide complete information on diagnoses of cases. Control selection requires equal thought and consideration, because the controls must come from the same population base as the cases; subtle differences in the way cases and controls are selected may lead to selection bias. The major point is that the controls have to reflect the population from which the cases arose. In a case-control study the challenge is to identify individuals that are
similar to the cases in all relevant respects except the exposure under study
(controls). Random-digit dialing (RDD) has often been the preferred source
of identifying population-based controls and it worked well until the mid
1990s. A 2.5 percent annual decline in the RDD response rates from 1982
to 2002 has been reported.The increasing use of cellular phones, caller identification, and multiple telephone numbers for a given household are a few of the emerging problems with RDD as a source of control selection today, and the potential exists for RDD control samples
to be biased with respect to socioeconomic status and population characteristics. control identification methods such as use of a friend,
neighborhood, family, or school controls have limitations that affect their
appropriateness in a study of cancer risks in populations near nuclear facili-
ties, including a high risk of overmatching on exposure and geographic lo-
cation. Additionally, school controls would be appropriate only for studies
of school-age children, and their use is likely to be administratively difficult
in a multistate study