Populations and samples

What is a population in statistics?

The whole set of items that are of interest

What does a census do?

Observes or measures every member of a population

What is a sample?

A selection of observations taken from a subset of the population which is used to find out information about the population as a whole

What is an advantage of using a census?

It should give a completely accurate result

What are the disadvantages of using a census?

• Time consuming and expensive

• Cannot be used when the testing process destroys the item

• It’s hard to process a large quantity of data

What are the advantages of using a sample?

• Less time consuming and expensive than a census

• Fewer people have to respond

• Less data to process than in a census

What are the disadvantages of using a sample?

• The data may not be as accurate

• The sample may not be large enough to give information about small sub-groups of the population

What are sampling units?

Individual units of a population

What is a sampling frame?

The list formed when sampling units of a population are individually named or numbered

What is random sampling?

When every member of the population has an equal chance of being selected

What are the three methods of random sampling?

• Simple random sampling

• Systematic sampling

• Stratified sampling

How is simple random sampling carried out?

Each person or thing is allocated a unique number and a selection of these numbers is chosen at random

What are the two methods used to choose the numbers in simple random sampling?

Generating random numbers (e.g. using a calculator) and lottery sampling

What happens in lottery sampling?

The members of the sampling frame could be written on tickets and placed into a ‘hat’. The required number of tickets would then be drawn out

What happens in systematic sampling?

The required elements are chosen at regular intervals from an ordered list- the first person should be chosen at random and you take at intervals from that starting point

What happens in stratified sampling?

The population is divided into mutually exclusive strata (e.g. males and females) and a random sample is taken from each

How do you calculate the number of people you should sample from each stratum?

(number in stratum/ number in population) x overall sample size

What happens in quota sampling?

An interviewer or researcher selects a sample that reflects the characteristics of the population

What happens in opportunity sampling?

A sample is taken from people who are available at the time the study is carried out and who fit the criteria you are looking for