Data collection

population

whole set of items that are of interest

census

observes or measures every member of a population

sample

selection of observations taken from a subset of the population which is used to find out information of the population as a whole

sampling units

individual units of population

sampling frame

sampling units of population individually named/ numbered to form list

simple random sample

every sample has equal chance of being selected

census advantages

should give completely accurate result

census disadvantages

• time consuming and expensive

• cannot be used when testing process destroys the item

• hard to process large quantity of data

sample advantages

• less time consuming and expensive than census

• fewer people have to respond

• less data to process than in census

sample disadvantgaes

• data may not be as accurate

• sample may not be large enough to give information about small sub-groups of population

systematic sampling

required elements chosen at regular intervals from ordered list

stratified sampling

population divided into mutually exclusive strata (e.g. males and females) and random sample taken from each

simple random sampling advantages

• free of bias

• easy and cheap to implement for small populations and small samples

• each sampling unit has known and equal chance of selection

simple random sampling disadvantages

• not suitable when population size or sample size is large as potentially time consuming, disruptive and expensive

• sampling frame needed (need list)

systematic sampling advantages

• simple and quick to use

• suitable for large samples and large populations

systematic sampling disadvantages

• sampling frame needed

• can introduce bias if sampling frame not random

stratified sampling advantages

• sample accurately reflects population structure

• guarantees proportional representation of groups within population

stratified sampling disadvantages

• population must be clearly classified into distinct strata

• selection within each stratum suffers from same disadvantages as simple random sampling

non-random sampling

quota sampling or opportunity sampling

quota sampling

interviewer/ researcher selects sample that reflects characteristics of whole population

opportunity sampling

sample from people who are available at time study carried out and who fit criteria you are looking for

quota sampling advantages

• allows small sample to still be representative of population

• no sampling frame required

• quick, easy and inexpensive

• allows for easy comparison between diff groups within population

quota sampling disadvantages

• non-random sampling → bias

• population must be divided into groups, can be costly/ inaccurate

• increasing scope of study increases no. of groups, adds time and expense

• non-responses not recorded as such

opportunity sampling advantages

• easy to carry out

• inexpensive

opportunity sampling disadvantages

• unlikely to provide representative sample

• highly dependent on individual researcher

quantitative data

associated with numerical observations

qualitative

associated with non-numerical observations

continuous variable

can take any value in given range

discrete variable

take only specific values in given range

Daily total rainfall

including solid precipitation e.g. snow and hail, melted before included in any measurements

• amount < 0.05mm recorded as ‘tr’ or ‘trace2’

daily mean wind direction and windspeed

• in knots

• ave over 24 hrs

• wind directions given as bearing and as cardinal (compass) directions

• windspeeds categorised according to Beaufort scale

daily max gust

• in knots

• highest instantaneous windspeed recorded

• direction also recorded

daily max relative humidity

• as % air saturation with water vapour

• >95% = misty and foggy conditions

daily mean cloud cover

• in ‘oktas’ or eighths of sky covered by cloud

daily mean visibility

• in decametres (Dm)

• greatest horizontal distance at which object can be seen

daily mean pressure

in hectopascals (hPA)