In the research, some individuals in the actual observation or investigation are called samples, and all the research objects are called populations. Sampling error refers to the error caused by the sampling method itself, that is, the difference between the sample index value and the inferred population index value.
According to the above definition, which of the followings does not involve sampling error?
To investigate the marriage satisfaction of urban residents across the country, 200 married people in Beijing were selected as the respondents