Data collection

Question 1

What is a population?

Answer 1

The population is the complete set of items you are interested in.

Question 2

What is a census?

Answer 2

A census measures a value from every member of the population.

Question 3

What is a sample?

Answer 3

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.

Question 4

Advantages of a census.

Answer 4

You get a completely accurate view of the population.

Question 5

Disadvantages of a census.

Answer 5

Time consuming and expensive
Cannot be used when the testing process destroys the items.
Not possible if the population is continually changing.

Question 6

Advantages of a sample.

Answer 6

Less time consuming and expensive than a census

Fewer people have to respond – so preferable when the population is large.

Question 7

Disadvantages of a sample.

Answer 7

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.

Question 8

What are sampling units?

Answer 8

Individual items of a population are known as sampling units.

Question 9

What is a parameter?

Answer 9

A parameter is a number that describes the entire population.

Question 10

What is a statistic?

Answer 10

A statistic is a number taken from a single sample – you can use one or more of these to estimate the parameter.

Question 11

What’s a sampling frame?

Answer 11

Often sampling units of a population are individually named or numbered to form a list called a sampling frame.

Question 12

What is random sampling?

Answer 12

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.

Question 13

What’s a simple random sample?

Answer 13

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.

Question 14

Advantages of a simple random sample.

Answer 14

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.

Question 15

Disadvantages of a simple random sample.

Answer 15

Not possible without a sampling frame. Potentially time consuming, disruptive and expensive when the population is large. Minority groups might be missed.

Question 16

What is a systematic sample?

Answer 16

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.

Question 17

Advantages of systematic sampling.

Answer 17

Can be quick and easy to use. Suitable for large samples and large populations.

Question 18

Disadvantages of systematic sampling.

Answer 18

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.

Question 19

What is a stratified sample?

Answer 19

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.

Question 20

Advantages of stratified sampling.

Answer 20

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.

Question 21

Disadvantages of stratified sampling.

Answer 21

Not possible without a sampling frame.
Strata must be carefully defined.
Sometimes difficult to split the population into naturally occurring groups.

Question 22

When is a sampling method bias?

Answer 22

A sampling method is biased if it creates a sample that does not represent the population.

Question 23

What is opportunity sampling?

Answer 23

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”.

Question 24

Advantages of opportunity sampling.

Answer 24

Easy to select the sample. Inexpensive.

Question 25

Disadvantages of opportunity sampling.

Answer 25

Unlikely to produce a sample representative of the population.
Highly dependent on the individual researcher, and who they think might be “nice” to interview!
This means the data collected can be biased.

Question 26

What is quota sampling?

Answer 26

Quota sampling is when the population is split into groups or strata as for stratified sampling (i.e that reflect the whole population). The size of each group determines the proportion of the sample that should have that characteristic.
Then a judgement is used to select the members from each group. The interviewer meets people, assesses their groups, and then once interviewed, allocates them into the appropriate quota.
This continues until all quota are filled. If someone refuses to be interviewed, or if the quota is already full, they are ignored, and the interviewer moves onto the next person.

Question 27

Advantages of quota sampling.

Answer 27

Even a small sample will should still be representative of the population. Does not need a sampling frame. Quick, easy, inexpensive.
Different group’s responses can be compared.

Question 28

Disadvantages of quota sampling.

Answer 28

Non-random, so could be biased.
The population must have been split into groups, which could be inaccurate or time consuming.
Non-responses aren’t recorded as such, which might distort interpretation.

Question 29

What is quantitative data?

Answer 29

Data associated with numerical observations.

Question 30

What is qualitative data?

Answer 30

Data associated with non-numerical observations.

Question 31

What is discrete data?

Answer 31

A type of quantitative data which only takes specific values in a given range (counted).

Question 32

What is continuous data?

Answer 32

A type of quantitative data which can take any value in a given range (measured).

Question 33

What does n/a mean in the large data set?

Answer 33

A reading that is not available.