Non-random sampling

What are the two types of non-random sampling?

Quota sampling and opportunity sampling

What does a quota mean?

A fixed share of something

What type of sampling do you use if you do not have a sampling frame?

Quota sampling

What is quota sampling?

When the population is divided into groups according to characteristics and then a quota of items in each group is set to try and reflect the group's proportion in the whole population

How do you carry out quota sampling?

What are three advantages of quota sampling?

• Allows a small sample to still be representative of the population.

• No sampling frame required.

• Allows for easy comparison between different groups within a population.

What are two disadvantages of quota sampling?

• Non-random sampling can introduce bias.

• Population must be divided into groups, which can be costly or inaccurate.

ESQ: Lake contains 3 species of fish - estimated to be 1400 trout, 600 bass and 450 pike. Survey of the health of the fish is carried out and a sample of 30 is chosen.

a) Give a reason as to why stratified sampling cannot be chosen?

The population is estimated so its impossible to obtain a sampling frame

b) State an appropriate sampling method for the survey?

Quota sampling

c) Give an advantage and disadvantage of this sampling method?

Advantage: sample can be obtained quickly.

Disadvantage: process is not random - possible bias.

d) Explain how this sampling method can be used to obtain a sample of 30 fish.

Work out the proportions of each fish which is needed:

Trout: 1400/2450 x 30 = 17.14

Bass: 600/2450 x 30 = 7.35

Pike: 450/2450 x 30 = 5.51. (2)

Fish are caught from the lake until the quota of 17 trout, 7 bass and 6 pike are filled. (1)

If a fish is caught and the species quota is full, then it is ignored. (1)

What is opportunity sampling?

Taking a sample of people who are available at the time that the study is being carried out and fit the criteria of the research.

How is opportunity sampling carried out?

The interviewer selects the actual sampling units according to the set criteria

What are two advantages of opportunity sampling?

• Easy to carry out.

• Inexpensive

What is the main disadvantage of opportunity sampling?

It is unlikely to provide a representative sample