Statistics - data collection (sampling)

Population

The whole set of items

Census

Observes or measures every member of the population

Sample

A selection of a subset (small amount) of the population

Sampling units

Individual units of a population (items on the list)

Sampling frame

list of individuals from which a sample is actually selected

Advantages of a census

Completely accurate result

Disadvantages of a census

Time consuming, expensive, large amount of data to process, uses up all the items

Advantages of a sample

Quick, cheap, less data to process

Disadvantages of a sample

Less accurate, small samples so may lack information about smaller groups

Simple random sampling

every member of the population has an equal probability of being selected for the sample

Examples of simple random sampling

names out of a hat, random number generator

Systematic sampling

Every nth item in the target population is selected from a random starting point

Stratified sampling

The population is divided into groups with a common attribute and a random sample is chosen within each group

Number sampled equation

number in group/number in population x overall sample size

Advantages of simple random sampling

Simple, no bias

Disadvantages of simple random sampling

Might miss information for small groups, sampling frame is needed - not suitable for large populations

Advantages of systematic sampling

Quick, simple, can be useful for large populations

Disadvantages of systematic sampling

Sampling frame may be needed, pattern in the list could be reflected in the results

Advantages of stratified sampling

Representative of all groups

Disadvantages of stratified sampling

Time consuming, simple random sample is needed afterwards - sample frame needed

Quota sampling

Researcher selects a sample which reflects the characteristics of the whole population

Opportunity sampling

Sample taken from items or people available at the time of study and who fit the criteria

Advantages of quota sampling

Small sample is still representative of whole population, no sampling frame required, quick, easy, cheap, allows easy comparison between different groups

Disadvantages of quota sampling

Non random so could be biased, dividing population can be costly or inaccurate, increasing sample size is time consuming and expensive, non responses are recorded

Advantages of opportunity sampling

Easy to carry out, cheap

Disadvantages of opportunity sampling

Unlikely to be representative, very dependent on the researcher

Qualitative data

Non numerical data

Quantitative data

Numerical data