In the research, a part of the individuals that are actually observed or surveyed are called samples, and the entire research object is called the population. Sampling error refers to the error caused by the sampling method itself, that is, the difference between the sample index value and the inferred overall index value. Based on the above definitions, which of the following is that do not involve sampling errors? ['Investigator misregisters sample data due to carelessness, negligence, etc.', 'Learn the loyalty of the public to a brand of shampoo by making random calls.', 'To understand the mathematics level of a class of students, statistical analysis of their math midterm exam results.', 'To survey the marriage satisfaction of urban residents nationwide, select 200 married people in Beijing as the survey object.']