11.2 Types of sampling

Question 1

What is sampling?

Answer 1

• Sampling means taking measurements of a limited number of individual organisms present in a particular area
• Sampling can be used to estimate the number of organisms in an area without having to count them all
• The number of individuals of a species present in an area is known as the abundance of an organism

Question 2

Random sampling

Answer 2

• Random sampling means selecting individuals by chance.
• In a random sample each individual in a population has an equal likelihood of selection, rather like picking names out of a hat
• To decide which organisms to study, random number tables or computers can be used.
• You have no involvement in deciding which organism to investigate.
• For example, to take random sample at a grass verge you could follow these steps:
  1) Mark out a grid on the grass using two tape measures laid at right angles
  2) Use random numbers to determine the x coordinate and the y coordinate on the grid
  3) Take a sample at each of the coordinate pairs generated

Question 3

Non - random sampling

Answer 3

• Opportunistic: this is the weakest form of sampling as it may not be representative of the population. It uses organisms that are conveniently available
• Stratified: Some populations can be divided into a number of strata based on a particular characteristic. For instance, the population might be separated into males and females. A random sample is then taken from each of these strata proportional to its size
• Systematic: in systematic sampling, different areas within an overall habitat are identified, which are then sampled separately. For example, systematic sampling may be used o study how plant species change as you move inland from the sea. Systematic sampling is often carried out using a line or a belt transect. A line transect involves marking a line along the ground between two poles and taking samples at specific points