Sample Strategies (Cambridge International IGCSE Environmental Management)

Random sampling

• Purpose of sampling

  • It gives an overview of the whole feature/population to be sampled
  • There is not enough time/equipment/access to measure the whole area being examined
  • Sampling provides a representative and statistically valid sample of the whole

• There are two different types of sampling

  • Random
  • Systematic

• What it is:

  • In random sampling, every point within a study area has an equal chance of being selected
  • Locations are usually chosen with a random-number generator, random coordinate tables, or by throwing a quadrat blindly

• How it works in practice:

  • Divide the site into a grid
  • Use random numbers to pick grid squares
  • Place quadrats or traps only in these selected positions

• Why it’s used:

  • It avoids bias and gives a more representative picture of species distribution and abundance

Systematic sampling

• What it is:

  • Samples are taken at regular, pre-determined intervals

    • For example every 500 meters or every tenth person

  • This could be along a transect line or at evenly spaced grid points

• How it works in practice:

  • Lay a tape measure across the habitat
  • Take samples every fixed distance (e.g. every 2 m)
  • Useful when there is an environmental gradient: e.g. changing light levels from woodland edge to interior or zonation on a rocky shore

• Why it’s used:

  • It reveals how populations change across space—ideal for detecting trends.
  • Avoids accidentally missing out sections of habitat due to chance

Benefits & limitations of sampling strategies

• All sampling methods have advantages and limitations

Random Sampling

• Best used for uniform habitats and broad population studies

Benefits

• Unbiased:

  • It reduces personal choice and avoids favouring particular areas

• Representative:

  • When enough samples are taken, it is useful for estimating population size and density
  • Can be used with a large sample area/population

• Good for uniform habitats, such as grasslands where species are spread fairly evenly

Limitations

• Time-consuming:

  • If the area is large and access to some random points is difficult

• May miss rare or patchy species:

  • Random points might not fall where those species actually occur
  • Less suitable for species with clumped distributions
  • Some sites selected may not be accessible or safe

• Requires accurate grid mapping; otherwise the randomness is compromised

Systematic Sampling

• Best used for transects, zonation studies, and environmental gradients

Benefits

• Excellent for environmental gradients:

  • Shows how species change along shorelines, woodland edges, polluted streams, etc.

• Easy to organise:

  • Fixed intervals make fieldwork efficient and repeatable

• Ensures full coverage of the transect or study area

  • It is easy and quick, making it more straightforward than random sampling

Limitations

• Can introduce bias:

  • Especially if the sampling interval coincides with a repeating pattern in the habitat (e.g. vegetation clumps every 2 m)

• Not always representative:

  • If species are distributed irregularly, the patterns may be missed between sampling points

• Less useful for estimating total population size

  • Especially for very mobile or randomly scattered species