Question

For each of the following scenarios, discuss (in at most five sentences) the main issue(s) with respect to sampling or reporting bias.

a) A particular city has 14 architects who own their own firm. To select a survey sample, each architect was contacted via telephone by order of appearance in the telephone directory, then the first 8 that agreed to be interviewed formed the sample.

b) The September 1992 issue of Prevention magazine included a women’s health survey; approx- imately 16,500 women responded to the survey. The May 1993 issue reported on the survey results, claiming that “92% of our readers rated their health as excellent, very good, or good”.

c) Many scholars and policymakers are interested in estimating the prevalence of mental illness among the homeless population. In one study, the authors sampled homeless persons who received medical attention from a clinic that was part of the Health Care for the Homeless project, resulting in an estimated prevalence of 33%.1 The authors maintain that selection bias is not a serious problem because the clinics are easily accessible to homeless people.

Answer 1

Part (a)

The telephone directory contains the name of the architect in order of name, thus while choosing 8 contacts of the architects from 14, only the 8 first architects who choose to take part are interviewed which reduces the probability of the architects who are further lower in the contact list, to be chosen. Thus the sampling technique is not random as every individual does not have equal probability of being selected.

Part (b)

The statement, “92% of our readers rated their health as excellent, very good, or good” is not statistically correct as the 16500 women who responded to the survey in the magazine are not the entirety of the readers of the magazine. Thus the sampling technique is not synonymous to the inference made.

Part (c)

The study, where the authors sampled homeless persons who received medical attention from a clinic that was part of the Health Care for the Homeless project, do not take into consideration, the homeless people who are not admitted or have not seeked help from the Health care centre or have attended a different health care centre or Homeless Project.