Chapter 1 - Data Collection

Population

Population

a collection of individual items (of interest)

What is a census

observation/measure of every member of a population

What is a sample?

A selection of observations taken from a subset of the population

What is the purpose of a sample?

To find out information about the population as a whole

What are the advantages of a census?

should give completely accurate result

What are the disadvantages of a census?

• time consuming and expensive

• cannot be used when testing as it destroys the product

• can be hard to process large quantities of data

What are the advantages of samples?

• less time consuming and expensive than a census

• fewer responses needed

• less data to process than a census

• not all products destroyed in testing

Disadv of samples

overall conclusion may not be as accurate
sample may not be large enough to include small sub-groups of a population

What are sampling units?

Individual units of a population

e.g. if sample uses university students, the sampling unit would be a single university student

What is a sampling frame?

A list of sampling units from which the sample is drawn. Each unit is given an identifying name or number.

What is random sampling?

Where every member in the sample has an equal chance of being chosen. It's representative of the whole population. Helps remove bias.

What are the methods of random sampling?

simple random sampling
systematic sampling
stratified sampling

What is simple random sampling?

Method where every sample unit has an equal chance of being chosen.
needs a sampling frame

Explain methods of simple random sampling

• Get sampling frame

• each member is allocated a number

• a selection of these numbers is chosen at random

• Methods of choosing the numbers are: random number generator, calculator or lottery sampling)

How to carry out Systematic Sampling?

• Starting point is randomly chosen

• required elements are chosen at regular intervals

e.g. sample size 20, elements are chosen every 5th member

What happens in Stratified sampling?

the population is divided into mutually exclusive groups and a random sample is taken from each.

number sampled in a stratum =

(Number in stratum/number in population) x overall sample size

Adv of simple random sampling

free from bias
fast, easy and cheap to implement
Each sampling unit has an equal chance of selection

Disadv of simple random sampling

Can introduce bias (especially if the population is large as it may not represent smaller subgroups of the population)
sampling frame needed

What are the advantages of systematic sampling?

simple and quick to use
suitable for large populations

What are the disadvantages of systematic sampling?

• A sampling frame is needed

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

What are the advantages of stratified sampling?

• Sample accurately reflects the population structure

• Guarantees proportional representation of groups within a population

What are the disadvantages of stratified sampling?

• population must be clearly classified into distinct stratum

• You need to know the structure of the population before you can take a stratified sample

• Classification into mutually exclusive strata may be difficult to implement

• selection in each stratum suffers same disadvantages of random sampling

Types of non-random sampling

Quota sampling and opportunity sampling

What is quota sampling?

Interviewer/researcher decides categories corresponding to the population and set a quota for each in proportion to their numbers in the population. Interview/test members until quotas are full.

opportunity sampling

Taking the sample from people who are available at the time the study is carried out and who fit the criteria of the study

What are the advantages of quota sampling?

• allows a small sample to still be representative of the population

• no sampling frame required

• quick, easy, inexpensive

• easy comparison between different groups within a population

What are the disadvantages of quota sampling?

• can introduce bias

• division into groups - costly, inaccurate

• increase size of study = more time in division = time consuming

What are the advantages of opportunity sampling?

• Easy to carry out

• Inexpensive

disadv of opportunity sampling

bias
unlikely to provide representative sample
highly dependent on individual researcher

Types of data

qualitative, quantitative
discrete, continuous

Class boundaries

The maximum and minimum values that belong in each class

What is the Large data set?

Weather data provided by the Met office for 5 UK weather stations and 3 overseas weather stations for May-October 1987 and 2015

5 uk weather stations

Leeming, Hurn, Camborne, Heathrow, Leuchars

3 overseas weather stations

Jacksonville, Florida
Beijing, China
Perth, Australia

Daily maximum temperature (UK only)

max temp reached over 24 hours

what temp is recorded overseas

daily mean temp

Daily total rainfall

includes solid precipitation (snow or hail)

any measurements less than 0.05mm recorded as trace/tr

daily total sunshine

Recorded to the nearest tenth of an hour

daily mean wind direction and windspeed

windspeed measured in knots (1.15 mph) and represented by the beaufort scale

wind directions - bearings and cardinal directions
both averaged over 24 hours from midnight to midnight

daily maximum gust

measured in knots
max instantaneous windspeed recorded

daily maximum relative humidity

as % air saturation

daily mean cloud cover

Measured in oktas or eighths of the sky covered by cloud

daily mean visibility

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

daily mean pressure

Measured in hectopascals (hPa)

Only things recorded overseas

daily avg temp
daily total rainfall
daily mean pressure
daily mean windspeed

Avg temp range and wind speed range in Camborne

Temp range - 10-20℃
Wind speed range - 3-18 kn

Avg temp range and wind speed range in Heathrow

Temp range: 8-29℃
Wind speed range - 3-19 kn

Avg temp range and wind speed range in Hurn

Temp range: 6-24℃
Wind speed: 2-19 kn

Avg temp range and wind speed range in Leeming

Temp range: 4-23℃
Wind speed: 2-17 kn

Avg temp range and wind speed range in Leuchars

Temp range: 4-19℃
Wind speed range - 3-23 kn

temp range and wind speed range in Beijing

Temp range: 8-33℃
Wind speed range - 2-9 kn

temp range and wind speed range in Jacksonville

Temp range: 15-31℃
Wind speed: 1-12 kn

temp range and wind speed range in Perth

Temp range: 8-25℃
Wind speed range - 4-14 kn

Station furthest north

Leuchars

Station furthest south

Perth

Inland stations

Heathrow, Leeming, Beijing

Coastal stations

Hurn, Camborne, Leuchars, Perth, Jacksonville

UK great storm

15-16th october 1987
gusts up to 100kn recorded

Florida Hurricanes

12 October 1987 - Hurricane Floyd
1-2 October 2015 - Hurricane Joaquin

Beaufort Scale

Scale 0 - calm - less than 1 knot
1-3 - light - 1 to 10 knots
4 - moderate - 11 to 16 knots
5 - fresh - 17 to 21 knots

What is lottery sampling?

Members of the sampling frame are written on a ticket and placed into a ‘hat’. The required number of tickets in then drawn from the hat.