Random and non-random sampling

What are the three types of random sampling methods

1. Simple random sampling

2. Systematic sampling

3. Stratified sampling

What is random sampling

• Each unit in the sample frame has an equal chance of being chosen

How do you carry out simple random sampling

1. Allocate number 1-n for each item

2. Use random number generator to select x DIFFERENT numbers from 1-n

3. Each item corresponds to number becomes the sample

Advantage of simple random sampling

• Bias free

• Easy and cheap for small sample/population

• Each unit has equal chance of being selected

Disadvantage of simple random sampling

• Not suitable for large population

• Need a sampling frame

What is systematic sampling

• Items are chosen at regular intervals in an ordered list

How do you carry out systematic sampling

• Select number from 1-n to start from

• Select every Kth item where K=population size/sample size

• Starting from the number chosen pick every kth number until you have picked x amount from list

Advantages of systematic sampling

• Quick to use

• Suitable for large sample/poplation

Disadvantage of systematic sampling

• Sample frame needed

• Can introduce bias if sample frame is not random

What is stratified sampling

• Population is divided into groups (known as strata)

• Simple random sampling is then carried out in each group

How to carry out stratified sampling

1. (Number in stratum/population size) x population

2. That is the percentage of people chosen from each strata then simple random sampling takes place

Advantages of stratified sampling

• Reflects population structure

• Proportional representation of each group

Disadvantage of stratified sampling

• Population must be clearly classified into distinct strata

What is a quota

• fixed share/number of items

What are the two types of quota sampling

• Quota sampling

• Opportunity sampling

What is quota sampling

• Selecting a sample that reflects characteristics of the whole population

How do you carry out quota sampling

1. Divide groups into characteristics of interest

2. Choose people until quota is filled

   • May use stratified sampling to get proportion of each group?

Advantage of quota sampling

• Small scale will still be representative

• No sampling frame required

• Quick, easy inexpensive

• Easy comparisons between different groups

Disadvantage of quota sampling

• Non-random could introduce bias

• Dividing population can be costly and inaccurate

What is opportunity sampling

• A sample taken from people who are available at the time of study who meet the criteria

How to carry out opportunity sampling

• Researcher will select unit based on set criteria

Advantages of opportunity sampling

• Easy to carry out

• Inexpensive

Disadvantage of opportunity sampling

• May not be representative

• Dependent on individual researcher