Causes of variation

In a completely randomised experiment,

Distinguishing the treatment effect and random variation

To find the effect of the treatments on the response, it is essentionl that we can distinguish it from random variation.

There must be enough data to estimate random variation separately from variation caused by the treatments.

Replication involves repeat measurements for each treatment. The variation within each treatment is all random variation.

Understanding the amount of random variation is necessary before you can interpret the effect of the treatments.