Sampling Strategies Flashcards
What is random sampling
Points are decided at random along a transect or within a set area.
Advantages of random sampling
Unbiased approach to sampling which reduces the potential of subjective choices of site locations.
Allows every site within the chosen sample area to have an equal chance of being chosen.
Disadvantages of random sampling
Random sites can lead to a clustering of sites being studied. This would lead inadvertently to an over-representation of one area.
Since anywhere can be chosen, this can lead to sites that ‘ought to be’ chosen being ignored.
Example of when random sampling would be used
To ask a random selection of pedestrians for a questionnaire when it makes no difference what age or gender they are.
What is systematic sampling
Points are chosen with equal divisions between each.
Advantages of systematic sampling
Acquires an equal spread of data across the entire area that has been chosen.
The sampling can easily be repeated at a later date since it is easy to accomplish and only requires the divisions in order to set the sample sites.
Recordings from different sample sets can be compared as their sampling strategy is identical.
Disadvantages of systematic sampling
The equal divisions dictate where the sites will be so unique sites of importance may be missed out if they do not fall on the site.
Sometimes impossible to collect data from where the systematic sample dictates eg a building on a linear transect line prevents traffic counts.
Examples of when systematic sampling would be used.
To test whether river characteristics develop downstream.
To study the change in urban characteristics as distance from the CBD increases.
What is stratified sampling
The chosen area is divided into categories (strata) using prior knowledge and a proportional number of sites are dedicated to each.
Advantages of stratified sampling
Allows prior knowledge to help inform where the sampling sites ought to be. This leads to a more informed representation of the study area.
Since a proportional number of sites are dedicated to each category this reflects the size of the entire sample site.
Disadvantages of stratified sampling
Stratified sampling requires prior knowledge of the site to determine where each category is.
Deciding upon the boundary for each category can be based upon personal opinion, which may lead to a subjective foundation for the sample site’s location.
Examples of stratified sampling
To decide where to place sample spots along a successional transect.
To decide where to put spots along an urban transect if the CBD, inner city and suburbs are located.
