Random sampling

Question 1

describe random sampling

Answer 1

every member of the population has an equal chance of being selected. The sampling should therefore be representative of the population it is selected from.

Question 2

what does random sampling help to remove?

Answer 2

bias

Question 3

what are the three methods of random sampling?

Answer 3

simple random sampling
systematic sampling
stratified sampling

Question 4

simple random sample

Answer 4

A sample of size n is one where every sample of size n has an equal chance of being selected.

Question 5

how do you carry out simple random sampling?

Answer 5

you need a sampling frame.
each person or item is given a unique number, and then a selection of these numbers is chosen at random using either a random number generator or lottery sampling

Question 6

Advantages of simple random sampling x3

Answer 6

free of bias
easy and cheap to do for small populations and samples
each sampling unit has an equal chance of selection

Question 7

disadvantages of simple random sampling x2

Answer 7

not suitable for large populations or samples

requires a sampling frame

Question 8

systematic sampling

Answer 8

the required sampling units are chosen at regular intervals

Question 9

how do you carry out systematic sampling?

Answer 9

e.g. if a sample is of size 40 was required from a population of 800, you would chose every 20th person, since 800/40=20.
The 1st person must be chosen at random by using a simple random sampling method, so e.g. the first 20 people on the list could be numbered and a random number generator used to select a starting point. you then select the 20th person along, and so on, from the starting point

Question 10

Advantages of systematic sampling x2

Answer 10

simple and quick to use

suitable for large populations and samples

Question 11

disadvantage of systematic sampling x2

Answer 11

sample frame required

can introduce bias if the sample frame is not random

Question 12

stratified sampling

Answer 12

the population is divided up into mutually exclusive strata and a random sample is taken from each.
the proportion in each strata should be the same

Question 13

how do you calculate the number of people per stratum (stratified sampling)?

Answer 13

(number in stratum/number in population) x sample size

Question 14

advantages of stratified sampling x2

Answer 14

sample accurately reflects the population structure

guarantees proportional representation of groups within a population

Question 15

disadvantages of stratified sampling x2

Answer 15

population must be able to be clearly classified into to distinct strata
selection within each strata suffers from the same disadvantages as simple random sampling