sampling

Question 1

what is random sampling?

Answer 1

equal chance of being chosen in order to avoid bias

e.g random number generator, names in a hat

Question 2

advantages of random sampling

Answer 2

• bias-free
• easy and cheap to implement
• equal chance of being selected

Question 3

disadvantages of random sampling

Answer 3

• not suitable for a large population size

- sampling frame needed

Question 4

what is systematic sampling?

Answer 4

• the required elements are chosen at regular intervals from an ordered list
• nth person

Question 5

how do we carry out a systematic sample?

Answer 5

nth = population size / sample size

e.g. 5th person starts from any random item between 1-5

Question 6

advantages of systematic sampling

Answer 6

• simple and quick to use

- suitable for large samples

Question 7

disadvantages of systematic sampling

Answer 7

• sampling frame needed

- can introduce bias if the sampling frame is not random

Question 8

what is stratified sampling?

Answer 8

a population divided into groups (strata) and a simple random sample carried out in each group

• used when the sample is large and the population naturally divides into groups e.g year groups

Question 9

how to carry out a stratified sample?

Answer 9

people in one group
—————————— X the sample needed
total population

Question 10

advantages of stratified sampling

Answer 10

• reflects population structure

- proportional representation of groups within the population

Question 11

disadvantages of a stratified sample

Answer 11

• population must be clearly classified into distinct strata

- selection within strata same disadvantages as simple random sampling