Unit One: data collection

Population

The whole set of items of interest

Census

Observes/measures every number of a population

Sample

Selection of observations taken from a subset of the population, used to find information about the population as a whole

Sampling unit

Individual units of a population

Sampling frame

The individually named/numbered sampling units to form a list

Simple Random Sampling: Random or not?

Random

Simple Random Sampling

One of every sample size has an equal chance of selection

Simple Random Sampling: Method

A sampling frame is needed (list of people/things)

Each individual is allocated a unique number and the selection of these numbers is chosen at random

Simple Random Sampling: method A

Generation random numbers using calculator, computer, or random number table

Simple Random Sampling: Method B

Lottery sampling

Members of sampling frame written on tickets placed into a ‘hat’. The required number of tickets are drawn out.

Simple Random Sampling: Advantages

Free of bias

Easy and cheap for small populations

Each sampling unit has known and equal chance of selection

Simple Random Sampling: Disadvantages

Not ideal when population size is large

Expensive, disruptive, time-consuming

Sampling frame needed

Systematic Sampling: Random or not?

Random

Systematic Sampling

The required elements are chosen at regular intervals from an ordered list

Systematic Sampling: Method

If a sample size of 20 was required from a popoulation of 100, you would take every fifth person (100/20=5)

Systematic Sampling: Advantages

• Simple and quick to use

• Suitable for large samples and populations

Systematic Sampling: Disadvantages

• A sampling frame is needed

• It can introduce bias if the sampling frame is not random

Stratified Sampling: Random or not

Random

Stratified Sampling

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

Stratified Sampling: Method

The proportion of each strata sampled should be the same. The formula to calculate the number of people sampled in a struatum is:
(Number of people in a stratum/number of people in population) x overall sample size

Stratified Sampling: Advantages

• Sample accurately reflects the population structure

• Guarantees proportional representation of groups within a population

Stratified Sampling: Disadvantages

• The population must be clearly defined into distinct strata

• Not ideal when population or sample size is too large

  • Expensive, disruptive, time consuming

• A sampling frame is needed

Quota Sampling: Random or not

Non-random

Quota Sampling

The interviewer/researcher selects a sample that reflects the characteristics of the whole population

Quota Sampling: Method

The population is divided into groups according to a given characteristic. The size of each group determines the proportion of the sample that should have the characteristic.

The interviewer meet people,assess their group, and then allocate them into the appropriate quota afterwards.

This continues until all quotas have been filled. If a person refuses to be interviewed or the quota is full, they are ignored.

Quota Sampling: Advantages

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

• No sampling frame is required

• Quick, easy, inexpensive

• Allows for easy comparison between different groups within a population

Quota Sampling: Disadvantages

• Non-random sampling can introduce bias

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

• Increasing scope of study increases number of groups

  • Time and expense

• Non-responses are not recorded

Opportunity (convenience) Sampling: Random or not?

Non-random

Opportunity (convenience) Sampling

Taking the sample from people who are available at the time the study is carried out and who fit the criteria

Opportunity (convenience) Sampling: Advantages

• Easy to carry out

• Inexpensive

Opportunity (convenience) Sampling: Disadvantages

• Unlikely to provide a representative sample

• Highly dependent on individual researcher

Population vs. Sample

Population