Statistics - Data Collection

Question 1

What is a population?

Answer 1

In statistics, a population is the whole set of items that are of interest.

Question 2

What is a census?

Answer 2

A census observes or measures every member of a 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 find out information about the population as a whole.

Question 4

What are the advantages and disadvantages of a census?

Answer 4

Advantages
- It 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 quantity of data.

Question 5

What are the advantages and disadvantages of a sample?

Answer 5

Advantages

• Less time consuming and expensive than a census.
• Fewer people have to respond.
• Less data to process than in 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.

Question 6

How can the size of the sample affect the validity of any conclusions drawn?

Answer 6

• The size of the sample depends on the required accuracy and available resources.
• Generally, the larger the sample, the more accurate it is, but you will need greater resources.
• If the population is very varied, you need a larger sample than if the population were uniform.
• Different samples can lead to different conclusions due to the natural variation in a population.

Question 7

What are sampling units?

Answer 7

Individual units of a population are known as sampling units.

Question 8

What is a sampling frame?

Answer 8

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

Question 9

What is random sampling?

Answer 9

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 10

What are the three methods of random sampling?

Answer 10

• simple random sampling
• systematic sampling
• stratified sampling

Question 11

What is simple random sampling?

Answer 11

A simple random sample of size ‘n’ is one where every sample of size ‘n’ has an equal chance of being selected.

Question 12

What is systematic sampling?

Answer 12

In systematic sampling, the required elements are chosen at regular intervals from an ordered list.

Question 13

What is stratified sampling?

Answer 13

In stratified sampling, the population is divided into mutually exclusive strata and a random sample is taken from each.

Question 14

What are the advantages and disadvantages of simple random sampling?

Answer 14

Advantages

• Free of bias.
• Easy and cheap to implement for small populations and small samples.
• Each sampling unit has a known and equal chance of selection.

Disadvantages

• Not suitable when the population size or the sample size is large.
• A sampling frame is needed.

Question 15

What are the advantages and disadvantages of systematic sampling?

Answer 15

Advantages

• Simple and quick to use.
• Suitable for large samples and large populations.

Disadvantages

• A sampling frame is needed.
• It can introduce bias if the sampling frame is not random.

Question 16

What are the advantages and disadvantages of stratified sampling?

Answer 16

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 the same disadvantages as simple random sampling.

Question 17

What are the two methods of non-random sampling?

Answer 17

• quota sampling

- opportunity sampling

Question 18

What is quota sampling?

Answer 18

In quota sampling, an interviewer or researcher selects a sample that reflects the characteristics of the whole population.

Question 19

What is opportunity sampling? (convenience sampling)

Answer 19

Opportunity sampling consists of 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.

Question 20

What are the advantages and disadvantages of quota sampling?

Answer 20

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

• Non-random sampling can introduce bias.
• Population must be divided into groups, which can be costly or inaccurate.
• Increasing scope of study increases number of groups, which adds time and expense.
• Non-responses are not recorded as such.

Question 21

What are the advantages and disadvantages of stratified sampling?

Answer 21

Advantages

• Easy to carry out.
• Inexpensive.

Disadvantages

• Unlikely to provide a representative sample.
• Highly dependent on individual researcher.

Question 22

What is quantitative data?

Answer 22

Variables or data associated with numerical observations are called quantitative variables or quantitative data.

Question 23

What is qualitative data?

Answer 23

Variables or data associated with non-numerical observations are called qualitative variables or qualitative data.

Question 24

What is a continuous variable?

Answer 24

A variable that can take any value in a given range is a continuous variable.

Question 25

What is a discrete variable?

Answer 25

A variable that can take only specific values in a given range is a discrete variable.

Question 26

What are classes?

Answer 26

Large amounts of data can be displayed in a frequency table or as grouped data.

When data is presented in a grouped frequency table, the specific data values are not shown. The groups are more commonly known as classes.

• Class boundaries tell you the maximum and minimum values that belong in each class.
• The midpoint is the average of the class boundaries.
• The class width is the difference between the upper and lower class boundaries.

Question 27

What is the daily mean temperature?

Answer 27

Daily mean temperature in degrees celsius - this is the average of the hourly temperature readings during a 24 hour period.

Question 28

What is the daily total rainfall?

Answer 28

Daily total rainfall including solid precipitation such as snow and hail, which is melted before being included in any measurements.

Question 29

What is the daily total sunshine?

Answer 29

Daily total sunshine recorded to the nearest tenth of an hour.

Question 30

What is the daily mean wind direction and windspeed?

Answer 30

Daily mean wind direction and windspeed in knots, averaged over 24 hours from midnight to midnight. Mean wind directions are given as bearings and as cardinal (compass) directions. The data for mean windspeed is also categorised according to the Beaufort scale.

Question 31

What is the daily maximum gust?

Answer 31

Daily maximum gust in knots - this is the highest instantaneous windspeed recorded. The direction from which the maximum gust was blowing is also recorded.

Question 32

What is the daily maximum relative humidity?

Answer 32

Daily maximum relative humidity, given as a percentage of air saturation with water vapour. Relative humidities above 95% give rise to misty and foggy conditions.

Question 33

What is the daily mean cloud cover?

Answer 33

Daily mean cloud cover measured in ‘oktas’ or eighths of the sky covered by cloud.

Question 34

What is the daily mean visibility?

Answer 34

Daily mean visibility measured in decametres (Dm). This is the greatest horizontal distance at which an object can be seen in daylight.

Question 35

What is the daily mean pressure?

Answer 35

Daily mean pressure measured in hectopascals (hPa).

Question 36

What are the similarities and differences between quota sampling and stratified random sampling?

Answer 36

A similarity between quota sampling and stratified random sampling is that the population is divided according to the characteristics of the whole population (into strata for stratified sampling, and groups for quota sampling). A difference between quota sampling and stratified random sampling is that quota sampling is non-random.