Statistical skills Flashcards
What makes a good sample?
It is precise
It is unbiased
It can be collected easily
It is large enough to provide conclusive results
What is stratified sampling?
Method is used when the parent population or sampling frame is made up of sub-sets of known size
Sub-sets make up different proportions and therefore sampling should be stratified to ensure that results are proportional and representative of the whole
What are the advantages of stratified sampling?
If the proportions of the sub-sets are known - it can generate results that are more representative of the whole population
Correlations and comparisons can be made between sub-sets
Very flexible
What are the disadvantages of stratified sampling?
Can be hard to stratify questionnaire data collection
Proportions of the sub-sets must be known and accurate if it is to work properly
May be heard to identify people’s ages effectively
What is random sampling?
Least biased of all sampling techniques
Can be obtained using random number tables
What are the advantages of random sampling?
Can be used with large sample populations - avoids bias
What are the advantages of random sampling?
Can lead to poor representation of the overall parent population or area if large areas are not hit my the random numbers generated
What is random point sampling?
A grid is drawn over a map of the study area
Random number tables are used to obtain coordinates references for the points
What is random line sampling?
Pairs of coordinates or grid references are obtained using random number tables and marked on the map
What is random area sampling?
A random number table generates coordinates or grid references which are used to make the bottom left corner of quadrats or grid squares to be sampled
What is systematic sampling?
Samples are chosen in a systematic or regular way
They are evenly distributed in a spatial context - e.g every 2 metres along a transect line
They can be at regular intervals
They can be regularly numbered
What are the advantages of systematic sampling?
It is more straight-forward than random sampling
A grid doesn’t have to be used
A good coverage of the study area can be more easily achieved than using random sampling
What are the disadvantages of systematic sampling?
It is more biased - not all members or points have an equal chance of being selected
It may lead to over or under-representation of a particular pattern
What is systematic point sampling?
Grind can be used and the points can be at intersections of the grid lines or in the middle of each grid square
Sampling is does along a transect line - e.g every 2 metres of every 10th pebble
What is systematic line sampling?
Eastings or northings of the grid on a map can be used to identify transect lines
