Chapter 1 - Data Collection

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62 Terms

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Population

a collection of individual items (of interest)

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What is a census

observation/measure of every member of a population

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What is a sample?

A selection of observations taken from a subset of the population

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What is the purpose of a sample?

To find out information about the population as a whole

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What are the advantages of a census?

should give completely accurate result

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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

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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

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Disadv of samples

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

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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

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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.

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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.

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What are the methods of random sampling?

simple random sampling
systematic sampling
stratified sampling

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What is simple random sampling?

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

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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)

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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

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What happens in Stratified sampling?

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

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number sampled in a stratum =

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

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Adv of simple random sampling

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

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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

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What are the advantages of systematic sampling?

simple and quick to use
suitable for large populations

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What are the disadvantages of systematic sampling?

  • A sampling frame is needed

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

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What are the advantages of stratified sampling?

  • Sample accurately reflects the population structure

  • Guarantees proportional representation of groups within a population

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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

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Types of non-random sampling

Quota sampling and opportunity sampling

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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.

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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

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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

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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

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What are the advantages of opportunity sampling?

  • Easy to carry out

  • Inexpensive

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disadv of opportunity sampling

bias
unlikely to provide representative sample
highly dependent on individual researcher

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Types of data

qualitative, quantitative
discrete, continuous

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Class boundaries

The maximum and minimum values that belong in each class

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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

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5 uk weather stations

Leeming, Hurn, Camborne, Heathrow, Leuchars

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3 overseas weather stations

Jacksonville, Florida
Beijing, China
Perth, Australia

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Daily maximum temperature (UK only)

max temp reached over 24 hours

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what temp is recorded overseas

daily mean temp

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Daily total rainfall

includes solid precipitation (snow or hail)

any measurements less than 0.05mm recorded as trace/tr

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daily total sunshine

Recorded to the nearest tenth of an hour

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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

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daily maximum gust

measured in knots
max instantaneous windspeed recorded

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daily maximum relative humidity

as % air saturation

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daily mean cloud cover

Measured in oktas or eighths of the sky covered by cloud

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daily mean visibility

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

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daily mean pressure

Measured in hectopascals (hPa)

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Only things recorded overseas

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

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Avg temp range and wind speed range in Camborne

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

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Avg temp range and wind speed range in Heathrow

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

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Avg temp range and wind speed range in Hurn

Temp range: 6-24℃
Wind speed: 2-19 kn

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Avg temp range and wind speed range in Leeming

Temp range: 4-23℃
Wind speed: 2-17 kn

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Avg temp range and wind speed range in Leuchars

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

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temp range and wind speed range in Beijing

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

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temp range and wind speed range in Jacksonville

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

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temp range and wind speed range in Perth

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

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Station furthest north

Leuchars

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Station furthest south

Perth

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Inland stations

Heathrow, Leeming, Beijing

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Coastal stations

Hurn, Camborne, Leuchars, Perth, Jacksonville

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UK great storm

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

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Florida Hurricanes

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

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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

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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.