Data collection Flashcards
(33 cards)
What is a population?
The population is the complete set of items you are interested in.
What is a census?
A census measures a value from every member of the population.
What is a sample?
A sample is a selection of observations taken from a subset of the population which is used to try to find out information about the population as a whole.
Advantages of a census.
You get a completely accurate view of the population.
Disadvantages of a census.
Time consuming and expensive
Cannot be used when the testing process destroys the items.
Not possible if the population is continually changing.
Advantages of a sample.
Less time consuming and expensive than a census
Fewer people have to respond – so preferable when the population is large.
Disadvantages of a sample.
The data may not be representative of the original population.
The sample may not be large enough to give information about small minority sub-groups of the population.
What are sampling units?
Individual items of a population are known as sampling units.
What is a parameter?
A parameter is a number that describes the entire population.
What is a statistic?
A statistic is a number taken from a single sample – you can use one or more of these to estimate the parameter.
What’s a sampling frame?
Often sampling units of a population are individually named or numbered to form a list called a sampling frame.
What is random sampling?
In random sampling, every member of the population has an equal chance of being selected. The sample should therefore be representative of the population.
Random sampling also helps to remove bias from a sample.
What’s a simple random sample?
A simple random sample of size n is one where every sample of size n has an equal chance of being selected.
To carry out a simple random sample, you need a sampling frame, usually a list of people or things. Each person or thing is allocated a unique number and a selection of these numbers is chosen at random.
E.g. lottery sampling or random number generator.
Advantages of a simple random sample.
Considered a fair way to select a sample.
Sample is probably representative of the population.
Each sampling unit has the same chance of being chosen.
Disadvantages of a simple random sample.
Not possible without a sampling frame. Potentially time consuming, disruptive and expensive when the population is large. Minority groups might be missed.
What is a systematic sample?
Systematic sampling is when you choose a starting point at random then systematically select objects a certain number apart.
For a population of 200 and a sample of 50: 200 = 4 50
Choose a random starting point from person 1 to 4 on the sampling frame (using RanInt#(1, 4)) then select every 4th member of the population until you have a sample of 50.
It is only random if the sampling frame has no order.
Advantages of systematic sampling.
Can be quick and easy to use. Suitable for large samples and large populations.
Disadvantages of systematic sampling.
Not possible without a sampling frame.
If the sampling technique coincides with a periodic trait in the population, the sampling technique will no longer be representative. This would introduce bias.
(E.g. sampling data across all 365 days of a year, using every 7th day could give you only data from Sundays, which might be different to the population as a whole)
There may be missing values in the population. Minority groups might be missed.
What is a stratified sample?
Stratified sampling is when the population is split into distinguishable groups which are quite different from each other and which together cover the whole population.
These groups are called strata. Within each group, or stratum, a sample is selected. The frequencies for each group in the sample are proportional
to the frequencies for each group in the population.
Advantages of stratified sampling.
Minimises sample selection bias by ensuring certain segments of the population are not overrepresented or underrepresented.
The frequencies for each sampled group can be proportional to the frequencies for each group in the population. Minor groups get included. Sample reflects the population.
Disadvantages of stratified sampling.
Not possible without a sampling frame.
Strata must be carefully defined.
Sometimes difficult to split the population into naturally occurring groups.
When is a sampling method bias?
A sampling method is biased if it creates a sample that does not represent the population.
What is opportunity sampling?
Opportunity sampling is the sampling technique most used by social science researchers. It consists of taking the sample from the target population who are available at the time the study is carried out and fit the criteria you are looking for.
This could be the first 20 people you meet outside a supermarket on a Monday morning who are carrying shopping bags. It could be “smokers”.
Advantages of opportunity sampling.
Easy to select the sample. Inexpensive.
