What is a census?
A census observes or measures every member of a population
What is a sample?
A sample is a selection of observations, taken from a subset of the population which is used to find out information about the population as a whole
What are the advantages and disadvantages of a census?
Advantages:
-should give a completely accurate result
Disadvantages:
-time consuming and expensive
-cannot be used when the testing process destroys the item
-hard to process large quantities of data
What are the advantages and disadvantages of a sample?
Advantages:
-less time consuming and expensive than a census
-fewer people have to respond
-less data to process than a census
Disadvantages:
-the data may not be as accurate
-the sample may not be large enough to give information about small sub-groups of the population
How does the size of the sample affect the validity of conclusions drawn?
-Larger sample = more accurate but you will need greater resources
-More varied population means you will need a larger sample than if the population were uniform
What are individual units of a population known as?
Sampling units
What is a sampling frame?
A list of all the members of a population, where you will choose the sample from.
Sampling units of a population are individually named or numbered to form this list.
What is a random sample?
A sample where every member has an equal chance of being selected
How do you carry out a simple random sample?
- Sampling frame- each member of a population is allocated a unique number
- Generate random numbers using a calculator or random number table, or pick numbers out of a hat (lottery sampling only works for small samples)
- Select the numbers chosen from the population
What is systematic sampling?
The required elements are chosen at regular intervals from an ordered list
How would you ensure that the first item chosen in a systematic sample is random?
Use a random number generator to generate a number between 1 and the interval (population size/sample size)
What is stratified sampling?
The population is divided into mutually exclusive strata (e.g males and females) and a random sample is taken from each
How do you calculate the number of people that should be sampled from each stratum?
Number samples in a stratum =
(Number in stratum / Number in population) x overall sample size
What are the advantages and disadvantages of simple random sampling?
Advantages:
-free of bias
-easy and cheap to implement for small populations and small samples
-sample is representative as each sampling unit has a known and equal chance of selection
Disadvantages:
-sampling frame is needed(not always easily obtainable)
-expensive and time consuming with large population sizes
What are the advantages and disadvantages of systematic sampling?
Advantages:
-simple and quick to use
-suitable for large populations and large samples
Disadvantages:
-sampling frame is needed
-can introduce bias if the sampling frame is not random
What are the advantages and disadvantages of stratified sampling?
Advantages:
-sample accurately reflects the population structure
-guarantees proportional representation of groups within a population
Disadvantages:
-population must be clearly classified into distinct strata
-selection within each stratum suffers from:
- sampling frame is needed (not always
easily obtainable)
-expensive and time consuming with
large population sizes
What are some types of random sampling?
Simple random sampling
Systematic sampling
Stratified sampling
What is quota sampling?
Population is grouped by characteristics and a fixed amount is sampled from every group
How is quota sampling carried out?
-Group population by characteristics (e.g. age and gender)
-Select quota (amount) for each group (e.g. 30 men under 25, 40 women over 30)
-Obtain sample by finding members of each group until the quotas are reached
What is opportunity (or convenience) sampling?
Taking the sample from people who are available at the time the study is carried out, and who fit the criteria you are looking for
What are the advantages and disadvantages of quota sampling?
Advantages:
-allows a small sample to still be representative of the population
-no sampling frame required
-quick, easy and inexpensive
-allows for easy comparison between different groups within a population
Disadvantages:
-can introduce bias
-population must be divided into groups which can be costly or inaccurate
What are the advantages and disadvantages of opportunity sampling?
Advantages:
-easy to carry out
-inexpensive
Disadvantages:
-unlikely to provide a representative sample
-highly dependent on individual researcher
What is the difference between qualitative and quantitative variables/data?
Qualitative = non-numerical observations
Quantitative = numerical observations
What is the difference between continuous and discrete data?
Continuous = a variable that can take any value in a given range
Discrete = a variable that can take only specific values in a given range
