Sampling

Question 1

What are the different sampling methods?

Answer 1

Random sample
Stratified
Quota sampling
Cluster
Systematic
Opportunity

Question 2

What is a random sample?

Answer 2

every sample of size n has the same probability of being selected - equal chance

Question 3

How would you choose a random sample of 40 when you have 400 students in a school?

Answer 3

Number students from 001 to 400
Select 3 digit random numbers
Ignore repeats, 000 and numbers greater than 400
Continue until 40 are obtained
Select corresponding students

Question 4

What is stratified sampling?

Answer 4

involves random sampling
the population is divided into appropriate categories + sample is chosen so the proportions in each category are the same as the proportions in the population

Question 5

If you have 400 students in a school with 60 in year 7, how do you work out how many year 7s to have in your sample of 40 students to make it fair?

Answer 5

60/400 x 40 = 6
number in year group / total number of students X sample number

Question 6

How would you do a stratified sample for year 7 (has 60 students) for a sample of 40?

Answer 6

for year 7
number students from 01 to 60
select 2 digit random numbers
ignore repeats, 00 and numbers greater than 60
continue until 6 are obtained
select corresponding students
do similar for other year groups

Question 7

What is Quota sampling?

Answer 7

Similar to stratified but doesn’t involve random sampling
Proportions are not necessarily the same as in the population

Question 8

How would you do quota sampling for year 7 (want 6 students) ?

Answer 8

for year 7
select 6 students conveniently
do similar for other year groups

Question 9

What is a cluster?

Answer 9

uses a natural division of the population
first chooses a random sample of clusters and then a sample from each cluster
usually contains random sampling

Question 10

How would you use cluster sampling when you want a sample of 40 using 2 clusters of 20?

Answer 10

randomly select 2 year groups
for example year 7 and year 10
for year 7
number students from 01 to 60
select 2 digit random numbers
ignore repeats, 00 and numbers greater than 60
continue until 20 are obtained
select corresponding students
do similar for year 10

Question 11

What is systematic sampling?

Answer 11

items are taken at regular intervals eg every 5th person in a list

Question 12

How would you use systematic sampling when you want a sample of 40 students but have a population of 400 students?

Answer 12

400/40 = 10 (total pop/number in sample)
number students from 001 to 400
randomly select a number from 001 to 010
then select every 10th student
continue until 40 are obtained
select corresponding students

Question 13

How would you use systematic sampling to get a sample of 30 when you have a population of 400?

Answer 13

400/30 = 13.33 (total pop / sample)
13 x 30 = 390
400-390=10, 10 left
13 + 10 = 23
number students from 001 to 400
randomly select a number from 001 to 023
then select every 13th student
continue until 30 are obtained
select corresponding students

Question 14

What is opportunity sampling?

Answer 14

selecting members of the population who are conveniently available + willing to take part in the study until you have a sample of the desired size