CIE A Level Biology (9700) 2019-2021

Revision Notes

18.1.3 Random Sampling

Random Sampling

  • Measuring the different levels of biodiversity within an ecosystem can be a tasking job
  • Finding out which species live in an ecosystem and the size of the populations requires the identification and cataloguing of all organisms present to build a species list
  • This is possible for areas that are very small or where the species are very large like trees
  • However, for larger and more complex ecosystems like rainforests, it is simply impossible to find, identify and count every organism that exists there
  • When this is the case different samples of the area can be taken and used to make an estimate for the total species numbers in the area


  • Sampling is a method of investigating the abundance and distribution of species and populations
  • There are two different types of sampling:
    • Random
    • Systematic
  • In random sampling the positions of the sampling points are completely random or due to chance
    • This method is beneficial because it means there will be no bias by the person that is carrying out the sampling that may affect the results
  • In systematic sampling the positions of the sampling points are chosen by the person carrying out the sampling
    • There is a possibility that the person choosing could show bias towards or against certain areas
    • Individuals may deliberately place the quadrats in areas with the least species as these will be easier and quicker to count
    • This is unrepresentative of the whole area
  • When a sampling area is reasonably uniform or has no clear pattern to the way the species are distributed then random sampling is the best choice

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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