Types of sampling

• what is sampling?

  taking measurements of a limited number of organisms present in a particular area
• what is random sampling?

  • selecting individuals by chance
  • Each individual has an equal likelihood of selection
• how is random sampling conducted?

  • mark out a grid in the area you want to sample
  • use random numbers (generated from a random number generator) to determine x and y coordinates on the grid
  • take a sample at each of the coordinate pairs generated
• what is non random sampling?

  where the sample is not chosen at random
• what are three types of non-random sampling methods?

  • opportunistic
  • stratified
  • systematic
• describe opportunistic sampling

  • weakest sampling method
  • as may not be representative of the population
  • uses organisms that conveniently available
• describe stratified sampling

  • populations divided into groups (strata) based on particular characteristics
  • random sample is then taken from each strata proportional to its size
• describe systematic sampling?

  • different areas within a overall habitat are identified, and then sampled separately
• what two pieces of equipment are used for systematic sampling?

  • line transect
  • belt transect
• what does a line transect involve?

  • marking a line along the ground between two poles
  • every organism that touches the line / is at a specified distance from the line is included in the sample
• what does a belt transect involve?

  • provides more information
  • two parallel lines are marked and samples are taken from the area between two lines
• why might a sample not be entirely representative of the organisms present in a habitat?

  • sampling bias- because of human involvement in sampling, may be on purpose or by accident eg- choosing an area that looks more interesting. Can be reduced by using random sampling
  • chance- organisms selected may, by chance, be not representative of the whole population. eg- five worms selected may be the five longest worms of that habitat. Effect can be minimised using a large sample size. Larger sample size means more reliable results