Sampling Flashcards
What is a simple random sample?
a random sample taken without replacement
What is an unrestricted random sample?
A random sample taken with replacement
Why take a random sample?
To avoid bias
How do you take a random sample?
Use random numbers
Method for a random sample
Allocate a random number from 01 to the number to each person
Move along random number table in pairs of digits
Ignore 00 and numbers > and repeats
Continue until * pairs of digits have been chosen
Select corresponding people
What is a stratified sample?
Requires prior knowledge to be used to divide the population into strata. Random samples are taken from each of these strata
Is each member of a Stratified sample likely to be chosen?
Yes if the random samples are taken in proportion to the size of each strata
Method for a stratified sample
Male/ total
Allocate a random number from 01 to category total
Move along random number table in pairs of digits
Ignore 00 and numbers > and repeats
Continue until pairs of digits have been chosen
Select corresponding *
Repeat for *
What is systematic sampling?
Members of the population are chosen at regular intervals
Method for a systematic sample
Population/ size. Every * person is selected
Choose a random number between 1 and…
Every * person after that
Equally likely to be chosen
Not a random sample as every possible sample cannot be chosen
What is quota sampling?
Stratified sample where the samples for each strata are chosen for convenience
Method for quota
Size/ total
What is a cluster sample?
Subgroups of diverse members
For geographical convenience
Is each member likely to be chosen in a cluster sample?
Each cluster has an equal chance but those in smaller clusters have a greater chance of being chosen
All members would be equally likely if all the branches had an equal amount of members
What is a random sample?
A sample selected in such a way that all possible samples of size n have an equal chance of being chosen
Advantages of a random sample
Simple and easy to use where small populations are involved
Sample is representative as it is randomly chosen
Negatives of a random sample
More time consuming and costly to carry out
Doesn’t ensure all strata are fairly represented
Advantages of a stratified random sample
Ensure all strata are fairly represented
Sample is representative as it is randomly chosen
Negatives of stratified random sample
You need prior knowledge
More difficult to administer
Advantages of systematic sample
Easier, quicker and cheaper to carry out
Can give useful results so long as the pattern of sampling doesn’t follow a pattern in the population
Negatives of systematic sample
Method may interact with a hidden pattern in the population leading to bias
Quota sample advantages
Easier, quicker and cheaper to carry out
Can give useful results when the stratification is skilfully carried out
Negatives of a quota sample
Interviewers will choose people easily accessible to themselves which may introduce bias
Cluster advantages
Easier to administer
Less costly in terms of traveling, time and money
Negatives of cluster sample
Subjects within the cluster may have similar views
Is stratified random a random sample
No, not all possible samples can be chosen
Is quota sample random?
No
Is systematic random?
No as not all subsets are possible, .eg 2 consecutive numbers can’t be in the sample sample
Do all systematic numbers have an equal chance of being chosen?
Yes, 1/? Chance if the allocation of random numbers has been done correctly
Are all subjects equally likely in a quota sample?
No as it depends on the convenient method used to choose subjects
Is quota random?
No
