In the Netherlands, all men take a military pre-induction exam at age 18. The exam includes an intelligence test known as "Ravenís progressive matrices," and includes questions about demographic variables like family size. A study was done to study the relationship between the test scores of 18-year-old men in the Netherlands in 1968 and the number of their brothers and sisters. The records of all the exams taken in 1968 were used. (a) What is the population? the sample? (b) Is there any sampling error? Explain brieáy.