Statistics: Sampling Methods

Population

Entire set of items in interest

Census

A sampling method where the entire population is observed/method

Sample

Subset of population is observed/measured

Census: Advantages (1) and disadvantages (3)

Advantages

• High accuracy

Disadvantages

• Can be time consuming if large volume of data to process

• Can be expensive

• Not always possible

Sampling frame

Complete list of entire population that is individually named or numbered

Sampling units

All different items in sampling frame

Simple random sampling method

1. Obtain sampling frame where each item is numbered

2. Generate random numbers to select items

3. Ignore repeated numbers and continue until desired sample size is reached

Simple random sampling: Advantages (2) and disadvantages (2)

Advantages:

• No bias

• Cheap and easy if population and sample size is small

Disadvantages:

• Requires a sampling frame

• Expensive and timely for larger populations/samples

Systematic sampling method

1. Obtain sampling frame and number each item

2. Divide population size by sample size to get K

3. Randomly select number between number and K and include this item

4. Sample every Kth item after this one

Systematic sampling: Advantages (2) and disadvantages (2)

Advantages:

• Simple and easy

• Good for large populations

Disadvantages:

• Requires a sampling frame

• If sampling frame isn’t random, then bias can be introduced

Stratified sampling method

1. Split population into groups (strata) based on characteristics

2. Determine the total sample size

3. Calculate the number needed from each strata proportionally

4. Sample required amount using random number generator

Stratified sampling: Advantages (2) and disadvantages (5)

Advantages:

• Sample structure reflects the population

• No groups are under or over represented

Disadvantages:

• Population must be split into distinct strata

• Classifying each item into each strata can be difficult

• Choosing which groups to include introduces bias

• Requires sampling frame

• Expensive or timely for larger populations and samples

Opportunity sampling method

1. Take samples from population of items at available at the time that fit the desired criteria

Opportunity sampling: Advantages (1) and disadvantages (3)

Advantages:

• Cheap and easy

Disadvantages:

• Not random introduces bias

• Dependent on researcher

• Not representative

Quota sampling method

1. Split population into groups based on characteristics

2. Determine the number (quota) of each group needed in sample (no need to be proportioned)

3. Gather items opportunistically and assign them into appropriate quota

4. Skip item if they fit all quotas

5. Continue until all quotes are full

Quota sampling: Advantages (4) and Disadvantages (4)

Advantages:

• Cheap and easy

• Sample structure reflects population

• No sampling frame required

• Easy comparison between groups

Disadvantages:

• Not random, so can be biased

• Not essential responses not recorded

• Dividing population into groups can be time costly

Continuous data

Data that must be measured

Quantitative data

Data given in numbers that counts or measures something