Stats T1 Flashcards
What happens in random sampling?
Every member of the population has an equal chance of being selected. The sample should therefore be representative of the population
What does random sampling remove?
It removes bias from the sample
What are the 3 methods of random sampling?
- Simple random sampling
- Systematic sampling
- Stratified sampling
What happens in simple random sampling?
A simple random sample size of n is one where every sample size n has an equal chance of being selected
What do you need to carry out a simple random sample?
You need a sampling frame, usually a list of people or things
Each person allocated a unique number & selection of these numbers is chosen at random
What are the two methods of choosing numbers in simple random sampling?
- Generating random numbers
- Lottery sampling
What is lottery sampling?
The members of the sampling frame could be written on tickets & placed into a ‘hat’
The required no. tickets would then be drawn out
What happens in systematic sampling?
The required elements are chosen at regular intervals from an ordered list
If a sample size od 20 was required from a population, how would you take the correct sample size?
You would take every fifth person
How must you choose the first person in systematic sampling?
They must be chosen at random
What happens in stratified sampling?
The population is divided into mutually exclusive strata (ie males and females) & a random sample is taken from each
What should the proportions be like from strata in stratified sampling?
The proportion of each strata should be the same
What is the formula used to work out the amount of people we should sample from each stratum?
No. in stratum
————————– x overall sample size
No. in population
What are the advantages of simple random sampling?
- Free of bias
- Easy & cheap to implement for small populations & small samples
- Each sampling unit has a known & equal chance of selection
What are the disadvantages of simple random sampling?
- Not suitable when the population size or the sample size is large
- A sampling frame is needed
What are the advantages of systematic sampling?
- Simple and quick to use
- Suitable for large samples & large populations
What are the disadvantages of systematic sampling?
- Sampling frame is needed
- It can introduce bias if the sampling frame is not random
What are the advantages of stratified sampling?
- Sample accurately reflects the population structure
- Guarantees proportional representation of groups within a population
What are the disadvantages of stratified sampling?
- Population must be clearly classified into distinct strata
- Selection within each stratum suffers from the same disadvantages as simple random sampling
What are the two types of non-random sampling?
- Quota sampling
- Opportunity sampling
What is quota sampling?
An interviewer/ researcher selects a sample that reflects the characteristics of the whole population
How is the population divided in quota sampling?
Divided into groups according to a given characteristic
What does the size of each group with a certain characteristic in quota sampling determine?
The size of each group determines the proportion of the sample that should have that characteristic
What is the interviewer’s role in quota sampling?
They would meet people, assess their group and then, after interview, allocate them into the appropriate quota