Sampling techniques

Random sampling

Random sampling

This method gives every member of the target group an equal chance of being selected for the sample (e.g. by assigning a number to each member, and then selecting from the pool at using a random number generator).

Strengths of random sampling

It is widely accepted that since each member has the same probability of being selected, there is a reasonable chance of achieving a representative sample.

Weaknesses of random sampling

• Small minority groups within your target group may distort results, even with a random sampling technique.

• It can be impractical (or not possible) to use a completely random technique, e.g. the target group may be too large to assign numbers to.

Systematic sampling

A systematic method is chosen for selecting from a target group, e.g. every fourth person in a list could be used in the sample. It differs from random sampling in that it does not give an equal chance of selection to each individual in the target group.

Strengths of systematic sampling

• Assuming the list order has been randomised, this method offers an unbiased chance of gaining a representative sample.

Weaknesses of systematic sampling

• If the list has been assembled in any other way, bias may be present. For example if every fourth person in the list was male, you would have only males in your sample.

Stratified sampling

Here the sampler divides or 'stratifies' the target group into sections, each showing a key characteristic which should be present in the final sample. Then each of those sections is sampled individually. The sample thus created should contain members from each key characteristic in a proportion representative of the target population.

Strengths of stratified sampling

• It avoids the problem of misrepresentation sometimes caused by purely random sampling.

Weaknesses of stratified sampling

• It takes more time and resources to plan.

• Care must be taken to ensure each key characteristic present in the population is selected across strata, otherwise this will design a biased sample.

Opportunity sampling

Participants who are both accessible and willing to take part are targeted, e.g. employees from a conveniently located employer near the laboratory could be selected for the sample group.

Strengths of opportunity sampling

• This method is easy and inexpensive to carry out.

Weaknesses of opportunity sampling

• The consequent sample may not be representative as it could be subject to bias (e.g. the conveniently located employer may undertake a selection process for job applicants, making it likely that employees possess certain similar characteristics that are unrepresentative of the wider target group).

Volunteer sampling

The sample consists of people who have volunteered to be in the study.

Strengths of volunteer sampling

• This often achieves a large sample size through reaching a wide audience, for example with online advertisements.

Weknesses of volunteer sampling

• Those who respond to the call for volunteers may all display similar characteristics (such as being more trusting or cooperative than those who did not apply) thus increasing the chances of yielding an unrepresentative sample.