SM1 - Population and Samples

Population

The whole set of items of interest

Sample

a subset of the population intended to represent it

Sampling Unit

item in a population

Sample advantages + disadvantages

• Cheap and quick

• Might not be accurate

• Sample might be too small

Census advantages + disadvantages

• Accurate

• Time consuming and expensive

• Large volumes of data need to be processed

Simple Random Sampling Method

• Create a sampling frame and number each sampling unit with a unique number.

• Use a random number generator to generate different numbers.

• Select corresponding students.

Simple Random Sampling advantages + disadvantages

• Bias free, easy and cheap

• Needs a sampling frame

• Not suitable for a large population

Systematic Sampling Method

• Use a random number generator to find a random number between I and k, where k is the population size / sample size.

• Select every k sampling unit.

Systematic Sampling advantages + disadvantages

• Quick and simple

• Suitable for larger populations

• Need sampling frame and can introduce bias if frame not random

Stratified Sampling Method

• Divide the population into strata.

• Number each member of each strata with a unique number.

• Use a random number generator to generate the same ratio of numbers as in the population, then match these to the corresponding sampling units.

Stratified Sampling advantages + disadvantages

• Reflects population structure so provides proportional representation.

• Must have clear strata.

• Need sampling frame.

Opportunity Sampling Method

• Take sample from available population.

Opportunity Sampling advantages + disadvantages

• Easy to carry out and inexpensive.

• No sampling frame.

• Bias means unlikely to provide representative sample.

Quota Sampling Method

• Population divided into groups according to characteristic.

• A quota of sampling units in each strata is chosen to reflect group’s proportion in the whole population.

• Select sampling units until every quota is full, ignoring units when their quota is full.

Quota Sampling advantages + disadvantages

• Doesn’t need sampling frame

• Can introduce bias if strata inaccurate.

• Representative of population.