What are the three methods of random sampling?
- Simple random sampling
- Systematic sampling
- Stratified sampling
How do you conduct a simple random sample size of n?
- 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
What is systematic sampling?
Required elements are chosen at regular intervals from an ordered list
An example of systematic sampling:
sample size is 20 required from 100
100 /20 = 5
first person chosen at random
every fifth person is chosen
What is startified sampling?
Population is divided into mutually exculsive strata (males and females)
random sample is taken from each
Formula for startified sample:
No of sampled in startum = No of startum/number in population *100
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)
- 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
*
Advantages of simple random sampling
- 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
Disadvantages of simple random sampling
- Not suitable when the population size or sample size is too large
- Sampling frame is needed
What is simple random sampling?
Sample size of n on where every sample size n has an equal chance of being selected
Disadvantages of systematic sampling
A sampling frame is needed
Can introduce bias if the sampling frame is not random
Advantages of stratified sampling
Sampling accurately reflects the population structure
Guraneetes the proportional representation of groups within a population
Disadvantages of stratified sampling
Populations must be classified inot a distinct strata
Selection within each startum suffers from same disadvantages as simple random samping
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?
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
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:
- 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
