The fact that some people fail to respond to a poll or respond only selectively to certain questions, ignoring others, can distort the accuracy of a survey. Market researchers call this nonresponse bias, and researchers at the Addiction Research Institute in Rotterdam, The Netherlands, concluded that it can be a serious problem. In fact, response rates to surveys in The Netherlands had dropped from 80 percent in the 1980s to 60 percent at the end of the 1990s, and were still continuing to decline, all leading to a smaller sample size and accuracy loss in population estimates. People who don’t respond to polls may have relevant characteristics different from responders.

In 2002, the researchers reviewed the results of a study done in 1999 on alcohol usage. Their research assumption was that abstainers probably didn’t respond because they lacked interest in the subject and excessive drinkers did not respond because they were embarrassed by their usage. This hypothesis was borne out in the subsequent study. In designing their study, they knew that nonresponse bias cannot be corrected simply by weighting data based on demographic variables. They needed to poll the nonrespondents and evaluate if their answers differed from responders. Originally, a random sample of 1,000 people, aged 16 to 69 years, was taken from the city registry of Rotterdam. Everyone received a mailed questionnaire about his or her alcohol consumption. After two reminders were sent, the response rate was 44 percent. For the follow-up study, the researchers chose 25 postal areas in Rotterdam and a secondary sample of 310 people. Of these, 133 had already responded to the first survey, and 177 did not, so these two groups were called primary respondents and primary nonrespondents, respectively. Members of the latter group were approached in person by the researchers in a series of five in-person attempts to conduct the interview at their homes. In the end, 48 primary nonrespondents could not be reached, leaving a final sample size for primary nonrespondents of 129.

Questions 

1. The nonresponse bias came in with the extremes (abstainers and heavy drinkers) regarding alcohol use. Is there any weighting approach that might compensate for these two important groups of nonresponders so that a follow-up study would not be needed?

2. For the 48 percent who failed to respond to the second study, would a mailed questionnaire, ensuring privacy, be worth the expense in terms of the improvement in statistical accuracy it might generate?