Q.Distinguish between a census (complete enumeration) and a sample survey, and state one situation where each is preferable.

The choice between studying every unit and studying a representative part is a key planning decision in any survey.

Census (complete enumeration). Every single unit of the field being studied is investigated. It gives the most complete and accurate picture, but it is costly, time-consuming and needs a large workforce, and is impractical when the field is very large or when testing destroys the item.

Sample survey. Only a representative part (a sample) of the field is studied, and conclusions about the whole are drawn from it. It is cheaper, faster and needs fewer resources, and is the only option when the field is huge or when testing is destructive — but it carries sampling error and depends heavily on the sample being genuinely representative.

When each is preferable.

• A census is preferred when the field is small, or when complete accuracy is essential and every unit matters — for example, a national population census. …

A common mistake is claiming a census is 'always more accurate' — it avoids sampling error but a badly conducted census can still be inaccurate, and a well-designed sample can be highly reliable. Al …