Guest Pollster | October 27, 2009
These comments are prompted by the paper Comparing the Accuracy of RDD Telephone Surveys and Internet Surveys Conducted with Non-Probability Samples by Yeager, Krosnick, et al, and by Mark Blumenthal's two excellent articles in the National Journal reviewing their paper.
The paper's conclusions were based on a comparison between six "benchmarks" and the findings of the various polls they examined. They assumed that the benchmarks were perfectly accurate, and that any differences between the polls and the benchmarks were "errors." I believe that this is not the case and that some of the benchmarks were inaccurate because of the social desirability bias that is often found in surveys where respondents are interviewed, by telephone or in-person, by live interviewers.
Social desirability bias occurs where respondents are not comfortable telling interviewers the truth because they are embarrassed to do so, or where their behavior or attitudes may be seen as unethical, immoral, anti-social or illegal.
Our online surveys have always found substantially more people than our telephone surveys who tell us they are gay, lesbian or bisexual (by a 3-to-1 margin). Our online surveys also find fewer people who claim to give money to charity, clean their teeth, believe in God, go to religious services, exercise regularly, abstain from alcohol, or drive under the speed limit.
Furthermore, in-person surveys by the Census Bureau report substantially more people claiming to have voted in elections than actually voted. If there is a better explanation than social desirability bias, I haven't heard it.
This conclusion - that surveys with live interviewers underreport "socially undesirable" behavior is supported by the data used by Yeager et al.
Our online survey, used by Yeager, found more smokers and more people having had 12 drinks in a life time than either the benchmark surveys conducted by government agencies or the RDD sample (and our own telephone surveys). Our online survey found that (to the nearest whole number) 28 percent were smokers compared to 26 percent in the RDD sample and 22 percent in the benchmark survey. Our online survey found only eight percent who had not had 12 drinks in their lifetime compared to 15 percent in the RDD sample and 23 percent in the benchmark survey.
Another government study, the NHANES study reported that 24.9 percent of adults said they were smokers but that blood tests showed that an additional 4.5 percent had smoked in the previous 24 hours but had not reported it when asked by an interviewer. The resulting NHANES estimate of 29 percent is closer to our estimate of 28 percent than to Knowledge Network's 26 percent or the RDD sample's 24 percent.
Two of the six benchmarks used by Yeager et al come from government sources where one would not expect to find any social desirability bias. In both cases, the Harris Interactive data were slightly closer to the benchmark data than were the findings of the RDD telephone survey. Our surveys found 28 percent of adults with passports compared to 30% for the RDD sample and the 23 percent in benchmark. Our survey found 92 percent having a driver's license compared to 93 percent in the RDD sample and the 89 percent benchmark.
In addition to the presence or absence of live interviewers there is one other reason why our online polls may have less social desirability bias than most telephone and in-person surveys. Our panel members have agreed in advance to be surveyed, which suggests that they trust us with confidential information, and are therefore more likely to tell the truth.
All this evidence suggests that the Harris Interactive data used by Yeager et al is generally more accurate than the RDD sample and that some of the so-called benchmarks probably overstate socially desirable behaviors because they were obtained in surveys with interviewers.