1.2: Sampling

Question 1

What are the three methods of random sampling?

Answer 1

• Simple random sampling
• Systematic sampling
• Stratified sampling

Question 2

How do you conduct a simple random sample size of n?

Answer 2

• Need sampling frame (list of people or things)
• Each person is allocated a unique number and selection of these numbers chosen at random
  
  • Two methods
  • Lottery sampling: (members of sampling frame written on tickets and placed in hat , then required number of tickets drawn out)
  • Or , generating random number using a calculator

Question 3

What is systematic sampling?

Answer 3

Required elements are chosen at regular intervals from an ordered list

Question 4

An example of systematic sampling:

Answer 4

sample size is 20 required from 100

100 /20 = 5

first person chosen at random

every fifth person is chosen

Question 5

What is startified sampling?

Answer 5

Population is divided into mutually exculsive strata (males and females)

random sample is taken from each

Question 6

Formula for startified sample:

Answer 6

No of sampled in startum = No of startum/number in population *100

Question 7

Example of startified sampling:

sample of 80

75 workers between 18 and 32

140 workers between 33 and 47

85 workers between 48 and 62

(name of method)

(explain how to use method to select sample of worker’s opnions)

Answer 7

• Startified sampling:
• 75 + 140 + 85 = 300 workers together
• 18 - 32 : 75/300 x 80 = 20 workers
• 33 - 47 : 140/300 x 80 = 37 workers
• 48-62: 85/300 x 80 = 23 workers

Number workers in each group and use random number sampling to produce quantity of random numbers and give questionaiire to workers with corresponding numbers

*

Question 8

Advantages of simple random sampling

Answer 8

• Free of bias
• Easy and cheap to implement for small populations and small samples
• Each sampling unit has a known and equal chance of selection

Question 9

Disadvantages of simple random sampling

Answer 9

• Not suitable when the population size or sample size is too large
• Sampling frame is needed

Question 11

What is simple random sampling?

Answer 11

Sample size of n on where every sample size n has an equal chance of being selected

Question 12

Disadvantages of systematic sampling

Answer 12

A sampling frame is needed

Can introduce bias if the sampling frame is not random

Question 13

Advantages of stratified sampling

Answer 13

Sampling accurately reflects the population structure

Guraneetes the proportional representation of groups within a population

Question 14

Disadvantages of stratified sampling

Answer 14

Populations must be classified inot a distinct strata

Selection within each startum suffers from same disadvantages as simple random samping

Question 15

A gym wans to take a sample of its member

Each member has a 5-digit membership number and gym wants to select every memeber with membership ending in 000

Is it systematic?

Answer 15

No

first person is not selected at random

required elements not chosen at regular intervals

could improve this by taking larger sample - reduce bias use simple random sampling

Question 16

A factory manager wants to get information about the ways their workers travel to work’

There are 480 workers in the factory and each has a clocking–in number

Number goes from 1 to 480

Explain how manager would take systematic sampling of size 30 of workers:

Answer 16

• 480/30 = 16
• Randomly select a number between 1 and 16
• Start with worker having clock-number then select every 16th clock number after this