Stats Year 1 Flashcards
(36 cards)
What is a population?
The whole set of items that are of interest
What is a sample?
The subset of the population intended to represent the population
What is a sampling unit?
Each individual thing in the population that can be sampled
What is a sampling frame?
A list of all of the sampling units
What is a census?
Data collected from the entire population
What are the advantages of a census?
Completely accurate/reliable
What are the disadvantages of a census?
Time consuming
Expensive
Large amount of data to process
Can not be used when testing involves destruction
What is qualitative data?
Non numerical values
What is quantitative data?
Numerical values
What is discrete data?
Can only take specific values
What is continuous data?
Can take any decimal values
How do you carry out a simple random sample?
-list all members of the population and assign them a number from 1-n
-use a random number generator to select k unique numbers
-select the corresponding members of the population which match the generated numbers to form the sample
What are the advantages of a simple random sample?
Easy/cheap
Each number has a known equal chance of being selected
Bias free
What are the disadvantages of simple random sampling?
Sampling frame needed
Not suitable when population is large
How do you carry out a systematic sample?
-take every K’th elements where
K=population size/sample size
-list all the members of the population and assign them a number from 1-n
-use a random number generator to select a number from 1-k as a starting point
-then select every k’th person to form the sample
What are the advantages of a systematic sample?
Simple and quick
Suitable for large samples
What are the disadvantages of systematic sampling?
Sample frame needed
Can introduce bias if sampling frame not random
How do you carry out a stratified sample?
-For each strata calculate…
(Size of strata/population size) x sample size
-then carry out a simple random sample for each strata
-repeat for each strata
What are the advantages of a stratified sample?
Reflects population structure
Guarantees proportional representation
What are the disadvantages of stratified sampling?
Not always east to classify into distinct strata
Not suitable when strata’s are too large
Sampling frame needed
How do you carry out a quota sample?
-population divided into groups according to a characteristic
-a quota of people/items in each group is set to try and reflect the groups proportion in the whole population
-interviewer selects actual sampling units
What are the advantages of quota sampling?
Representative of whole populations
No sampling frame required
Quick/easy/inexpensive
Allows comparison between different groups in population
What are the disadvantages of quota sampling?
Non random sampling can introduce bias
Population must be divided into groups = costly/inaccurate
Increasing scope of study increased number of groups, adding time/expense
Non responses are not recorded
How do you carry out an opportunity sample?
Sample taken from people who are available at time of study, who meet criteria
