Chapter 2 Flashcards
What is a population?
The whole set of items that are of interest.
What is a census?
A census observes or measures every member of a population.
What is a sample?
A selection of observations taken from a sub-set of the population.
What is a sample survey?
A sample used to find out information about the population as a whole.
Advantages of a census
Should give a completely accurate result.
Disadvantages of a census
Time consuming and expensive. Difficult to process information as there is so much of it. Cannot be used when testing to destruction.
Advantages of a sample survey.
Costs less and quicker than a census. Fewer people have to respond in the sample. Less data to deal with.
Disadvantages of a sample survey.
Data may not be as accurate. Sample may not be large enough to give information about small sub-groups of the population.
What is a random sample?
A sample in which every possible sample of size n has an equal chance of being selected.
What is a sampling frame?
A list identifying every single sampling unit that could be included in the sample.
What is raw data?
The information obtained from a census or sample survey.
When would a census be used?
When the population is very small and extreme accuracy is required.
What is unrestricted random sampling?
Random sampling with replacement.
What are the advantages of simple random sampling?
Provided that the population is small, it is cheap, simple and standard formulae can be used to analyse the results. Also each person/unit is included only once.
What are the disadvantages of simple random sampling?
It is not suitable when the population is large, or when a sampling frame is required.
What are two simple techniques for simple random sampling?
Random number sampling and lottery/ticket sampling.
What are the advantages of random number sampling?
The numbers are truly random and free from bias, It is easy to use and each number has a known and equal chance of being selected.
What is the disadvantage of random number sampling?
Not suitable for large populations.
What are the advantages of lottery sampling?
Tickets are drawn at random, it is easy to use and each ticket has a known chance of selection.
What are the disadvantages of lottery sampling?
It is not suitable where the population is large and a sampling frame is needed.
What is systematic sampling?
The required elements are chosen at regular intervals from an ordered list. Select first unit randomly.
How do you calculate a systematic sample?
Take every kth element from a sampling frame where k=N/n N=population size and n=sample size.
When is systematic sampling used?
When the population is too large for simple random number sampling.
What are the advantages of systematic sampling?
It is simple to use and suitable for large samples.
What are the disadvantages of systematic sampling?
It is only random if the ordered list is truly random and it can introduce bias.
What is stratified sampling?
A form of random sampling in which the population is divided into mutually exclusive groups (strata) and a random sample is taken from each. The same proportion of each stratum is taken in the sample as is found in the population.
How do you calculate the number sampled in a stratum?
It is the number in the stratum divided by the size of the population multiplied by the overall sample size.
What are the advantages of stratified sampling?
It can give more accurate estimates than simple random sampling where there are clear strata present and it reflects the population structure.
What are the disadvantages of stratified sampling?
Within the strata, the problems are the same as for any simple random sample. If the strata are not clearly defined they may overlap.
When is non-random sampling used?
When it is not possible to use random methods, for example when no sampling frame is available.
Explain quota sampling.
The population is divided up into groups in terms of gender, social class etc. The number of people in each group is set to try and reflect the group’s proportion in the whole population. The interviewer selects the actual sampling units.
What are the advantages of quota sampling?
It enables the fieldwork to be done quickly because a representative sample can be achieved with a small sample size. Costs are kept to a minimum. Administering the test is easy.
What are the disadvantages of quota sampling?
It is not possible to estimate the sampling errors as it is not a random process. The interviewer has to choose the respondents and may not be able to judge the characteristics easily. Non-responses are not recorded. It can introduce interviewer bias in who is included.
What is primary data?
Data which is collected by or on behalf of the person who is going to use it.
What is secondary data?
Data which is neither collected by nor on behalf of the person who is going to use it.
What are the advantages of primary data?
The collection method is known. The accuracy is known. The exact data needed are collected.
What are the disadvantages of primary data?
It is costly in time and effort.
What are the advantages of secondary data?
They are cheap to obtain (government publications for example). A large quantity of information is available on the Internet. Much of the data has been collected for years and can be used to plot trends.
What are the disadvantages of secondary data?
Bias is not always recognized. It can be in a form that is not easy to deal with.
