Sampling Flashcards
What is random sampling?
Locations chosen by chance. E.g. using a random number generator
What are the strengths of random sampling?
Removes human bias when selecting of sites/population - where may want to select sites that confirm hypotheses, Everything has an equal chance of being selected.
What are the weaknesses of random sampling?
If sample size is small it may be unrepresentative result. No guarantee that the sample will be representative of the population.
What is systematic sampling?
Locations chosen at equal intervals. E.g. ask every 5th person who passes you.
What are the strengths of systematic sampling?
Accurately shows continuous changes. E.g. velocity across a river / people along a road.
Can be more accurate than random sampling as avoids the chance that a random sampling only selects from one part of a population.
What are the weaknesses of systematic sampling?
Can pick bias as sampling every 50cm or so may be crossing point for pedestrians etc.
Access at regular intervals may be impossible (e.g. due to private land/unsafe)
What is stratified sampling?
Total population divided into smaller groups (strata) an equal proportion to those in the total population are used. E.g. measure traffic on different road types.
What is the strength of stratified sampling?
Ensures all areas/types/categories are represented.
What are the weaknesses of stratified sampling?
Difficult to know which subsets of data to include without a pilot study or prior use of secondary data.
Access to specific sites may be difficult.
What is opportunistic sampling?
Selecting anyone who is available and willing to take part.
What are the strengths of opportunistic sampling?
Less time consuming; easiest method to use. Allows access to sites that are safe/ on public land.
What are the weaknesses of opportunistic sampling?
Biased. Data is unlikely to accurately represent the population.
What is transect sampling?
Data collected along a line. E.g. biodiversity across a footpath, environmental quality across the city centre.
What is the advantage of transect sampling?
Shows continuous change.
What is the weakness of transect sampling?
Can pick bias in transect location.
