Sampling methods

Question 1

Population

Answer 1

Whole group of items of interest

Question 2

Census

Answer 2

A survey of the whole population

Question 3

Sample

Answer 3

Subset of the whole population, ideally chosen randomly and therefore unbiased

Question 4

Sampling unit

Answer 4

Each individual member of a population

Question 5

Sampling frame

Answer 5

Named/Numbered list of all members of the population

Question 6

Random sampling

Answer 6

Sampling method where every member of the population has an equal chance of being selected
Needs sampling frame
Sample produced should be unbiased and representative of population

Question 7

Non-random sampling

Answer 7

Sampling method that is not random
Some members of population are more likely to be chosen than others
May be biased

Question 8

What is simple random sampling

Answer 8

Every member of the population has an equal chance of being selected

Question 9

Simple random sampling method

Answer 9

In sampling frame each unit has identifying number
Use random number generator or lottery sampling (names in a hat)

Question 10

Advantages and disadvantages of simple random sampling

Answer 10

Advantages
-Bias free and representative
-Easy and cheap to implement
-Each number has equal chance of being selected

Disadvantages
-Sampling frame needed
-Not suitable when population size is large
-Possibly time consuming and expensive for larger sample/population
-Might be missing values in sample (e.g. data not available for particular day)

Question 11

What is systematic sampling

Answer 11

Required elements are chosen at regular intervals in ordered list

Question 12

Systematic sampling method

Answer 12

Take every nth element
n = POPULATION SIZE/SAMPLE SIZE

Question 13

Advantages and disadvantages of systematic sampling

Answer 13

Advantages
-Simple and quick
-Should be unbiased and representative as is random
-Better than simple random for large samples/populations

Disadvantages
-Sampling frame needed
-Can introduce bias if sampling frame not random (e.g. selecting every 7th value in a list of consecutive dates)
-May be missing values in sample

Question 14

Systematic example
Telephone directory contains 50k names
Researcher wishes to select systematic sample of 100 names from it
Explain how researcher should obtain sample

Answer 14

Get sampling frame by allocating number to each name
Divide 50000 by 100 to find 500
Random number generator for starting number between 1-500
Select every 500th element after starting number until 100 names obtained

Question 15

What is stratified sampling

Answer 15

Population divided into groups and a simple random sample carried out in each group
Used when sample is large and population naturally divides into groups

Question 16

What are the groups in stratified called

Answer 16

Strata

Question 17

Stratified example
6th form wants to survey students to get view on new centre
Year 12 = 140 boys, 160 girls
Year 13 = 118 boys, 132 girls
Will sample 110 students
Explain how to get a stratified sample

Answer 17

Calculate total number of students
Calculate each proportion with number/total x 110, e.g. 140/550 X 110 = 28
Then to select 28 out of 140 boys allocate number 1-140 and randomly generate 28 numbers with corresponding names
Same for other 3 strata

Question 18

Stratified method

Answer 18

(NUMBER IN GROUP/TOTAL) X SAMPLE SIZE

Question 19

Advantages and disadvantages of stratified sampling

Answer 19

Advantages
-Sample accurately reflects population structure
-Guarantees proportional representation of groups within population
-Random selection within each strata

Disadvantages
-Needs sampling frame
-Population must be clearly classified into distinct strata which may be difficult
-Selection within each stratum suffers from same disadvantages as simple random sampling

Question 20

What is quota sampling

Answer 20

Population divided into groups according to characteristic
A quota of items/people in each group is set to try and reflect the group’s proportion in the whole population
Interviewer selects the actual sampling units

Question 21

Advantages and disadvantages of quota sampling

Answer 21

Advantages
-Allows small sample to still be representative of population
-No sampling frame required
-Quick, easy, inexpensive
-Allows for easy comparison between different groups in population

Disadvantages
-Non-random sampling can introduce bias
-Population must be divided into groups, which can be costly or inaccurate
-Increasing scope of study increases number of groups, adding time/expense
-Non-responses are not recorded

Question 22

Quota example
Assume we know 70% of people shopping in Kingston are female and 30% are male
Explain how to take sample of 200 shopping using quota sampling method

Answer 22

70%x200=140 females
30%x400=60 males
Interviewer would then choose and interview shoppers until she had 140 females and 60 males that fit her quota

Question 23

What is opportunity/convenience sampling

Answer 23

Sample taken from people who are available at time of study, who meet criteria

Question 24

Advantages and disadvantages of opportunity/convenience sampling

Answer 24

Advantages
-Easy to carry out
-Inexpensive

Disadvantages
–Unlikely to provide a representative sample - likely to be biased
-Highly dependent on individual researcher

Question 25

Example of opportunity/convenience sampling

Answer 25

Standing outside Bentalls centre in Kingston and surveying the first 20 teenagers available on e.g. exit polls at a polling station to determine likely election result