Sampling

Question 1

Target population

Answer 1

Population to whom findings are generalisable

Question 2

Study population

Answer 2

The proportion of the target population available to the researcher. Collected using a sampling frame.

(Representativeness, completeness)

Question 3

Most commonly used sampling frames

Answer 3

• cancer registers
• GP lists
• professional registers (GMC, nursing & midwifery council)
• workplaces, schools, hospital clinics

Question 4

What are the drawbacks of using the UK electoral register as a sampling frame?

Answer 4

Under-representation:
- young/students
- travellers
- homeless

Question 5

Why do we use sampling?

Answer 5

Because investigating the entire population of interest is usually costly & impractical.

Question 6

Name 3 methods of random sampling

Answer 6

1. Simple random
2. Cluster
3. Stratified

Question 7

Simple random sampling

Answer 7

From list of whole study population, draw a random sample. (ie- random number generator - equal probability of being chosen)

Question 8

Cluster sampling

Answer 8

Random selection from within natural clusters in a population (schools, GP practices, households)

Often used in RCTs to reduce contamination btw patients

Question 9

Stratified sampling

Answer 9

When measurement of interest is likely to vary btw different sub-groups, one may selectively recruit a higher proportion of some minority groups to increase statistical power and sample size

(equal chance of being picked but random higher proportion of one group)

Question 10

Name 4 non-random sampling methods

Answer 10

1. Systematic sampling
2. Convenience sampling
3. Snowball sampling
4. Street survey

Interpretation - avoid inferences about gen pop

Question 11

Systematic sampling

Answer 11

Pick every nth individual from a sampling frame (potential for bias if underlying patterns in sampling frame)

Question 12

Convenience sampling

Answer 12

Choose study ptpts on basis of being readily available.

(Subject to volunteer bias)

Question 13

Snowball sampling

Answer 13

Ask study ptpts to recommend acquaintances who meet pre-specified criteria

(Useful when studying hard-to-reach groups when no sampling frame is available)

Question 14

Street survey

Answer 14

Pick ppl who appear to fall into certain categories of interest from the street & ask your Qs

(Often quota sampling - specify certain number M/F from certain age groups)

No sampling frame, difficult to know who findings applicable to