Sampling Flashcards
Define a census?
Measures every member of the population
Advantages of a census
1) It should give a completely accurate result
Disadvantages of a census
1) Time consuming & expensive
2) Cannot be used when the testing process destroys the item
3) Hard to process large quantity of data
Define a sample
A selection of observations taken from a subset of the population which is used to find out information about the population as a whole
Advantages of a sample
1) Less time consuming & expensive than a census
2) Fewer people have to respond
3) Less data to process than in a census
Disadvantages of a sample
1) The data may not be as accurate
2) The sample may not be large enough to give information about small sub-groups of the population
Define sampling unit
Individual units of a population.
Define sampling frame
Sampling units given a name or number and compiled into a list
Define quantitative data
You can associate data with numerical observations
Define qualitative data
Associate data with non-numerical observations
Summarise simple random sampling
Every sample of size n has an equal chance of being selected
Describe how simple random sampling is conducted
1) Create a sampling frame
2) Pick each sample by lottery or random number generator
Advantages of simple random sampling
1) Free of bias
2) Easy and cheap to implement for small populations & small samples
3) Each sampling unit has a known and equal chance of selection
Summarise systematic random sampling
Required elements are chosen at regular intervals from an ordered list
Disadvantages of simple random sampling
1) Not suitable when the population size or the sample size is large
2) A sampling frame is needed
Describe how systematic random sampling is conducted
1) Create an ordered list
2) If sample of 20 required from 100, divide 100 by 20 = 5
3) Starting at any random number, record that first then every 5th value e.g 2, 7, 12
Summarise stratified random sampling
Population divided into mutually exclusive strata (e.g males and females) and a simple random sample is taken from each
Describe how you conduct stratified random sampling
1) Divide population into mutually exclusive strata
2) Keep proportion the same by: number in strata/number in population x sample size
3) Round to nearest whole number
4) Perform simple random sampling
Advantages of stratified random sampling
1) Sample accuracy reflects the population structure
2) Guarantees proportional representation of groups within a population
Disadvantages of stratified random sampling
1) population must be clearly classified into distinct strata
2) Selection within each stratum suffers from the same disadvantages as simple random sampling
Summarise the non-random sampling, Quota sampling
An interviewer/researcher select a sample that reflects the characteristics of the whole population
Describe how the non random sampling, Quota sampling, is conducted
1) Design characteristic groups and divide population into these groups
2) Interview each person and assess which group they fall into
3) Size of each group determines proportion of the sample that should have that characteristic
4) Stop interviewing when quota field. People who refuse to be interviewed are ignored, move onto next person
Advantages of the non-random sampling, quota sampling
1) Allows a small sample to still be representative of the population
2) No sampling frame required
3) Quick, easy, inexpensive
4) Allows for easy comparison between different groups within the population
Disadvantages of the non random sampling, quota sampling
1) Non-random sampling can introduce bias
2) Population divided into groups which is costly or inaccurate
3) Increasing scope of study increases number of groups, adding time and cost
4) Non-responses are not recorded
