Exploration & description/Fixed and random effects

A random effect is a component of the data that has a degree of randomness associated with it, whereas a fixed effect has no random connotation.

An example of a fixed effect in this case study would be the sex of a lamb. It is fixed because it can only have one of two values: male and female. 

On the other hand, the influence of the ram on the growth of its offspring is usually considered to be a random effect. In making this assumption the researcher assumes that the sample of rams used in the study is a random selection of rams from the particular genotype at large.