Sampling methods Flashcards
Population
Whole group of items of interest
Census
A survey of the whole population
Sample
Subset of the whole population, ideally chosen randomly and therefore unbiased
Sampling unit
Each individual member of a population
Sampling frame
Named/Numbered list of all members of the population
Random sampling
Sampling method where every member of the population has an equal chance of being selected
Needs sampling frame
Sample produced should be unbiased and representative of population
Non-random sampling
Sampling method that is not random
Some members of population are more likely to be chosen than others
May be biased
What is simple random sampling
Every member of the population has an equal chance of being selected
Simple random sampling method
In sampling frame each unit has identifying number
Use random number generator or lottery sampling (names in a hat)
Advantages and disadvantages of simple random sampling
Advantages
-Bias free and representative
-Easy and cheap to implement
-Each number has equal chance of being selected
Disadvantages
-Sampling frame needed
-Not suitable when population size is large
-Possibly time consuming and expensive for larger sample/population
-Might be missing values in sample (e.g. data not available for particular day)
What is systematic sampling
Required elements are chosen at regular intervals in ordered list
Systematic sampling method
Take every nth element
n = POPULATION SIZE/SAMPLE SIZE
Advantages and disadvantages of systematic sampling
Advantages
-Simple and quick
-Should be unbiased and representative as is random
-Better than simple random for large samples/populations
Disadvantages
-Sampling frame needed
-Can introduce bias if sampling frame not random (e.g. selecting every 7th value in a list of consecutive dates)
-May be missing values in sample
Systematic example
Telephone directory contains 50k names
Researcher wishes to select systematic sample of 100 names from it
Explain how researcher should obtain sample
Get sampling frame by allocating number to each name
Divide 50000 by 100 to find 500
Random number generator for starting number between 1-500
Select every 500th element after starting number until 100 names obtained
What is stratified sampling
Population divided into groups and a simple random sample carried out in each group
Used when sample is large and population naturally divides into groups
What are the groups in stratified called
Strata
Stratified example
6th form wants to survey students to get view on new centre
Year 12 = 140 boys, 160 girls
Year 13 = 118 boys, 132 girls
Will sample 110 students
Explain how to get a stratified sample
Calculate total number of students
Calculate each proportion with number/total x 110, e.g. 140/550 X 110 = 28
Then to select 28 out of 140 boys allocate number 1-140 and randomly generate 28 numbers with corresponding names
Same for other 3 strata
Stratified method
(NUMBER IN GROUP/TOTAL) X SAMPLE SIZE
Advantages and disadvantages of stratified sampling
Advantages
-Sample accurately reflects population structure
-Guarantees proportional representation of groups within population
-Random selection within each strata
Disadvantages
-Needs sampling frame
-Population must be clearly classified into distinct strata which may be difficult
-Selection within each stratum suffers from same disadvantages as simple random sampling
What is quota sampling
Population divided into groups according to characteristic
A quota of items/people in each group is set to try and reflect the group’s proportion in the whole population
Interviewer selects the actual sampling units
Advantages and disadvantages of quota sampling
Advantages
-Allows small sample to still be representative of population
-No sampling frame required
-Quick, easy, inexpensive
-Allows for easy comparison between different groups in population
Disadvantages
-Non-random sampling can introduce bias
-Population must be divided into groups, which can be costly or inaccurate
-Increasing scope of study increases number of groups, adding time/expense
-Non-responses are not recorded
Quota example
Assume we know 70% of people shopping in Kingston are female and 30% are male
Explain how to take sample of 200 shopping using quota sampling method
70%x200=140 females
30%x400=60 males
Interviewer would then choose and interview shoppers until she had 140 females and 60 males that fit her quota
What is opportunity/convenience sampling
Sample taken from people who are available at time of study, who meet criteria
Advantages and disadvantages of opportunity/convenience sampling
Advantages
-Easy to carry out
-Inexpensive
Disadvantages
–Unlikely to provide a representative sample - likely to be biased
-Highly dependent on individual researcher
