Some survey researchers recognized the value of data from non-asking sources. They are to be commended! The only way to know if self-reports are correct or incorrect is to check or verify them with data from non-asking sources. Newsletter No. 97, which I received from and university survey research unit, describes how biomeasures can be used to check or verify self-reports. In bold are the parts of Newsletter No. 97 most relevant to this point.
Using Survey Data in Combination with Data from Other Sources: Incorporating Biomeasures with Survey Data
In this [Newsletter], we consider one particular type of data that might be collected
along with survey data: biomeasures. Although surveys predominantly rely
on data from self-reports, it is increasingly common to collect other sorts of
data as well. For example, particularly in health surveys, researchers may collect biomeasures.
Biomeasures are physical measures taken by the survey interviewer
or a researcher either at the time of the survey interview or (in some
cases) at a later time during a physical exam. Examples of biomeasures collected with surveys include direct measures such as height, weight, waist/hip circumference, blood pressure; simple physical performance tests (walking, balance, strength, cognition); or collection of specimens such as saliva, urine, or blood samples.
There are several reasons why researchers may incorporate biomeasures with
survey data: 1) to make population-representative inferences, if the survey
sample is randomly drawn, 2) to serve as a reference to self-reported
behaviors and health measures, 3) to better understand causal links between social /
environmental exposures and health, and 4) to explore the role of genetics.
Biomeasures may be particularly valuable when they provide data for variables that are difficult to assess via self-report, either because respondents may not be able to accurately answer survey questions on the topic (e.g., current blood pressure) or because the questions may be sensitive (e.g., illegal drug use). . . .”
For more information about sensitive questions, respondent cooperation/response rate, and other aspects of survey research, see The Problem with Survey Research.
See also my blog post, Counter Literature to Survey Research.