Why is it often desirable to take samples?

Answer 1

Samples are often used as all possible data could be impractically large to procure.

For instance, if I wanted to find out the average weight of frogs in a lake, I could drain the entire lake, use a chemical spray to immobilise all frogs, and then weigh each one individually, but that would be a mammoth task. Fencing off a section of the lake and taking a sample would be considerably cheaper, less time consuming, and more practical, and would only give us a slightly less accurate result.

Another issue is that people often are unwilling/unable to respond to a survey. If we tried to get the shopping habits of everyone going into a shop, many will be in a hurry or just plain not want to be surveyed. For this reason, the results will end up being incomplete anyway, so the person may as well set clear qualities she wants a sample to have.

Answer 2

Samples are often used as all possible data could be impractically large to procure.

For instance, if I wanted to find out the average weight of frogs in a lake, I could drain the entire lake, use a chemical spray to immobilise all frogs, and then weigh each one individually, but that would be a mammoth task. Fencing off a section of the lake and taking a sample would be considerably cheaper, less time consuming, and more practical, and would only give us a slightly less accurate result.

Another issue is that people often are unwilling/unable to respond to a survey. If we tried to get the shopping habits of everyone going into a shop, many will be in a hurry or just plain not want to be surveyed. For this reason, the results will end up being incomplete anyway, so the person may as well set clear qualities she wants a sample to have.