Sampling

• Random sampling

  A sample that fairly represents a population because each member has an equal chance of inclusion
• How to take a random sample

  1. Mark out a grid on the grass using 2 tape measures at right angles

  2. Use random numbers to determine the x-coordinate and y-coordinate on your grid

  3. Take a sample at each of the coordinate pairs generated
• Non-random sampling


  An alternative sampling method to random sampling, where the sample is not chosen at random. It can be opportunistic, stratified, or systematic
• Opportunistic sampling (NRS)


  Sampling using the organisms that are conveniently available. The weakest form of sampling as it may not be representative of the population
• Stratified sampling (NRS)


  The population is divided into a number of strata (subgroups) and weighted based on a particular characteristic
• Systematic sampling (NRS)

  Select some starting point and then select every kth element in the population
• Line transect- systematic sampling


  A line is marked along the ground and samples are taken at specified points
• Belt transect- systematic sampling


  2 parallel lines are marked, and samples are taken of the area between the two lines
• Sampling bias


  A problem that occurs when a sample is not representative of the population from which it is drawn
• Chance


  The organisms selected may, by chance, not be representative of the whole population
• Pooters- sampling technique


  Used to catch small insects. Suck on a mouthpiece and insects are drawn into the holding chamber via the inlet tube
• Sweep nets- sampling technique


  Used to catch insects in areas of long grass
• Pitfall traps- sampling technique


  Used to catch small, crawling invertebrates. A hole is dug in the ground which insects fall into with a roof left overnight
• Tree beating - sampling technique


  Used to take samples of the invertebrates living in a tree or bush. A large white cloth is stretched out under the tree, the tree is shaken/beaten to dislodge the invertebrates onto the cloth
• Kick sampling- sampling technique

  Used to study organisms living in a river. The river bank or bed is 'kicked' for a period of time to disturb the substrate, a net is held just downstream for a set period of time in order to capture any organisms released into the flowing water
• Point quadrat (Plant sampling)

  A frame has a horizontal bar - set intervals along the bar long pins can be pushed through the bar to reach the ground, each plant that touches the pin is recorded
• Frame quadrat- density

  Count the number of individual large plants in a 1m x 1m square quadrat. This gives you the density per square meter. It is an absolute measure
• Frame quadrat- frequency

  Using the small grids within a quadrat, count the number of squares a particular species is present in
• Frame quadrat- percentage cover


  It is an estimate by eye of the area within a quadrat that a particular plant species covers. It is fast