Chapter 1 - Data Collection

Question 1

What is a population?

Answer 1

The whole set of items that are of interest (the thing being surveyed is the population)

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

Census advantages:

Answer 4

• Gives a completely accurately result
• Representative of everyone (and smaller subgroups)

Question 5

Census disadvantages:

Answer 5

• Time consuming
• Expensive
• Cannot be used when the testing process destroys the item
• Hard to process large quantities of data

Question 6

Sample advantages:

Answer 6

• Less time consuming
• Less expensive
• Fewer people have to respond
• Less data to process

Question 7

Sample disadvantages:

Answer 7

• The data may be less accurate
• The sample may not be large enough to represent small subgroups
• The results could be biased

Question 8

What are individual units of a population known as?

Answer 8

Sampling units (i.e. a person in a larger survey is a sampling unit)

Question 9

What is a sampling frame?

Answer 9

A list of individually numbered or named sampling units of a population

Question 10

What does the size of a sample depend on?

Answer 10

The required accuracy and available resources

Question 11

How is the validity of a sample affected by its size?

Answer 11

• Generally, the larger the sample, the more accurate it is (but you will require greater resources)
• If the population is 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 12

Why do we random sample?

Answer 12

We randomly sample because it means every member of the population has an equal chance of being selected. The sample should therefore be representative of the population. It also helps to remove bias from the sample

Question 13

What are the 3 methods of random sampling?

Answer 13

• Simple random sampling
• Stratified sampling
• Systematic sampling

Question 14

Simple random sampling:

Answer 14

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. Selections can be made using random number generators or lottery style sampling (e.g. pulled from a hat)

Question 15

Stratified sampling:

Answer 15

In stratified sampling, the population is divided into mutually exclusive strata (distinct subgroups of the population e.g. males and females) and a random sample is taken from each. The number selected from each stratum is reflective of the proportion of that stratum within the population

Question 16

Systematic sampling:

Answer 16

In systematic sampling, the required sampling units are selected from an ordered list, and made at regular, chosen intervals

The size of the interval depends upon the number in the population, as well as the number desired from the sample. Divide the population by the sample number and round down, then use this value as the difference between the ordered terms, after selecting a starting point less than this value

Question 17

Advantages of simple random sampling:

Answer 17

• 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

Question 18

Disadvantages of simple random sampling:

Answer 18

• Not suitable when the population size or the sample size is large as it is potentially time consuming, disruptive and expensive
• A sampling frame is needed
• Could exclude minorities

Question 19

Advantages of Systematic sampling:

Answer 19

• Simple and quick to use
• Suitable for large samples and large populations
• Cheap and easy

Question 20

Disadvantages of Systematic sampling:

Answer 20

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

Question 21

Advantages of Stratified sampling:

Answer 21

• Sample accurately reflects the population structure
• Guarantees proportional representation of groups within a population
• Small sample sizes required - saves time and money

Question 22

Disadvantages of Stratified sampling:

Answer 22

• Population must be clearly classified into distinct strata
• Selection within each stratum suffers from the same disadvantages as simple random sampling
• The requirement of a sampling frame means it’s time consuming and expensive

Question 23

What are the 2 types of non-random sampling?

Answer 23

• Quota Sampling
• Opportunity Sampling

Question 24

Quota Sampling:

Answer 24

In quota sampling, the interviewer/researcher first determines the different characteristics of the populations that they wish to represent. These will be mutually exclusive in the same way that the strata for a stratified sample are

It is then determined how many people you wish to question from each group. (this can be determined in the same manner as a stratified sample)

As an interviewer, you would then meet members of the population, assess which strata they fall into, and then allocate them into the appropriate quota

Once you have met your quota for a group, you no longer include any further members into that group

You continue this process until your quota for each group is filled

Question 25

Opportunity sampling:

Answer 25

Opportunity sampling consists of taking the sample from people who are available at the time of the study, and who fit the relevant criteria For example, if you wish to find out the purchasing habits of shoppers from a particular store, you may choose to question those who are leaving the store and have made a purchase

Question 26

Advantages of Quota sampling:

Answer 26

• 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

Question 27

Disadvantages of Quota sampling:

Answer 27

• 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 - leads to bias

Question 28

Advantages of Opportunity sampling:

Answer 28

- Easy - Cheap - Quick - No sampling frame required

Question 29

Disadvantages of Opportunity sampling:

Answer 29

- Unlikely to provide a representative sample - Highly dependent on the individual researcher

Question 30

Answer 30

Question 31

What is quantitative data?

Answer 31

Variables or data associated with numerical observations are called quantitative variables or quantitative data For example, you can give a number to shoe size so shoe size is a quantitative variable

Question 32

What is qualitative data?

Answer 32

Variables or data associated with non-numerical observations are called qualitative variables or qualitative data For example, you can't give a number to hair colour (blonde, red, brunette). Hair colour is a qualitative variable

Question 33

What is continuous variable?

Answer 33

A variable that can take any value in a given range is a continuous variable For example, time can take any value, e.g. 2 seconds, 2.1 seconds, 2.01 seconds etc.

Question 34

What is a discrete variable?

Answer 34

A variable that can take only specific values in a given range is a discrete variable For example, the number of girls in a family is a discrete variable as you can't have 2.65 girls in a family

Question 35

Grouped frequency tables:

Answer 35

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 36

LDS - Daily mean temperature:

Answer 36

Daily mean temperature in °C - this is the average of the hourly temperature readings during a 24-hour period

Question 37

LDS - Daily total rainfall:

Answer 37

Daily total rainfall including solid precipitation such as snow and hail, which is melted before being included in any measurements - amounts **less than 0.05 mm** are recorded as **'tr' or 'trace'** ‘tr’ means 0 in an exam question

Question 38

LDS - Daily total sunshine:

Answer 38

Daily total sunshine recorded to the nearest tenth of an hour

Question 39

LDS - Daily mean wind direction and windspeed:

Answer 39

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 40

LDS - Beaufort scale:

Answer 40

Question 41

LDS - Daily maximum gust:

Answer 41

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 42

LDS - Daily maximum relative humidity:

Answer 42

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 43

LDS - Daily mean cloud cover:

Answer 43

Daily mean cloud cover measured in 'oktas' or eighths of the sky covered by cloud

Question 44

LDS - Daily mean visibility:

Answer 44

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

Question 45

LDS - Daily mean pressure:

Answer 45

Daily mean pressure measured in hectopascals (hPa)

Question 46

How are missing values from the Large Data Set represented?

Answer 46

n/a or ‘not available’