Sampling bias is when the samples chosen for a study are not representative of the whole population. 

It is important to avoid sampling bias to obtain representative samples.

The best way to prevent sampling bias is to eliminate any human involvement in choosing the samples, by carrying out random sampling.

This involves taking measurements of individuals selected randomly from the population of organisms that is being investigated.

We can minimise the effect of chance in random sampling by using a large sample size and analysing the data collected.

1. Mean - the sum of the sampled values divided by the number of values
2. Median - the central or middle value of a set of values
3. Mode - the single value of a sample that occurs most often

A normal distribution curve is bell-shaped and symmetrical around a central mean value.

It is typical for a feature that shows continuous variation.

The standard deviation is a measure of the range of values either side of the mean.

The mean is the measurement at the maximum height of the curve. It provides an average value and is useful for comparing one sample with another.

Interspecific variation refers to the differences between different species.

Intraspecific variation refers to the differences between members of the same species.