Non-random Sampling

This lesson covers: 

  1. Differences between random and non-random sampling
  2. Types of non-random sampling
  3. Non-random sampling techniques
  4. Measuring abiotic factors

Random and non-random sampling

Non-random sampling involves selection based on specific criteria or patterns and the sample is not chosen at random.


Differences between random and non-random sampling:

  1. Random sampling removes human bias in sample selection, improving reliability.
  2. Non-random sampling does not use random selection so is prone to bias.

Types of non-random sampling

There are three main types of non-random sampling.


  1. Opportunistic:
  • This uses conveniently available organisms.
  • It may not be representative of the population.


  1. Stratified:
  • This divides the population into subgroups (strata) based on a characteristic.
  • A random sample is taken from each subgroup/stratum.
  • It ensures all strata of the population are represented proportionally in the sample.


  1. Systematic:
  • This samples different areas in a habitat separately at regular intervals to avoid bias.

Line transects and belt transects

Transect sampling is a method of studying the distribution of organisms in a specific area. It is typically used in systematic sampling.


Transect sampling:

  • Mark a line (transect).
  • Take samples along this line at regular intervals.


This lets us study how species' distributions change across different areas within a habitat, like from a woodland to a lake.

Illustration showing line transect and belt transect methods for sampling organisms in a habitat, with examples from woodland to lake.

There are two main types of transect sampling:

  • Line transect - Samples are taken at regular intervals along a line between two points.
  • Belt transect - Samples are taken in an area along a line or between two parallel lines, by using quadrats placed either side by side (continuous) or at regular intervals (interrupted).

Measuring abiotic factors

Abiotic factors directly affect organisms in a habitat so they are useful to measure when sampling.


We can measure abiotic factors like:

  • Light, humidity, and temperature (using sensors).
  • pH and wind speed (using probes).
  • Dissolved oxygen (using specialised probes).


Advantages of measuring abiotic factors:

  • They can detect rapid changes.
  • They can reduce human error in taking readings.
  • They can achieve a high degree of precision
  • They allow data to be stored and tracked on a computer.