Statistics - data collection

Population definition

whole set of items that are of interest

sample definition

some subset of population intended to represent population

sampling unit

each individual thing in the population that can be sampled is known as sampling unit

sampling frame

often sampling units of a population are individually named / numbered to form list called sampling frame

Census - advantages + disadvantages

Advantages:

• should give completely accurate result

Disadvantages:

• time consuming and expensive

• can not be used when testing involves destruction

• large volume of data to process

Sample - advantages + disadvantages

Advantages:

• cheaper

• quicker

• less data to process

Disadvantages:

• data may not be accurate

• data may not be large enough to represent small sub-groups

Simple random sampling - method:

• number pop

• use RNG to select a set amount of numbers w/ no repetition

• find the ppl corresponding to numbers

Simple random sampling - advantages and disadvantages

advantages:

• no bias

• easy + cheap

Disadvantages:

• sampling frame needed

systematic sampling - method:

• number pop

• take every kth value where k=pop/sample

• find the ppl corresponding to numbers

systematic sampling - advantages and disadvantages:

advantages:

• easy

• good for big pop

disadvantages:

• can introduce bias

• sampling frame needed

stratified sampling - method:

• split pop into groups / strata

• work out proportional sample for each group / strata

• simple random sampling w/in each strata

stratified sampling - advantages and disadvantages:

Advantages:

• proportional representation (mimics population)

Disadvantages:

• good strata needs to be chosen

quota sampling - method:

• divide pop by needed quality

• then randomly sample

quota sampling - advantages and disadvantages:

Advantages:

• quick

• easy

• allows for comparison between groups

Disadvantages:

• bias

• expensive

opportunity sampling - method:

• anyone who is available + willing

opportunity sampling - advantages and disadvantages

Advantages:

• easy

• cheap

Disadvantages:

• unlikely to be representative

• very dependent on the researcher

Types of Data - Qualitative:

non-numerical vales e.g. colour

Types of Data - Quantitative:

numerical values

Discrete vales

can only take specific values, e.g. shoe size, number of children

Continues values

can only take any decimal value (possible with a specific range)