All statistical significance tests start with a null hypothesis. A statistical significance test measures the strength of evidence that the data sample supplies for or against some proposition of interest.
This proposition is known as a 'null hypothesis', since it usually relates to there being 'no difference' between groups' or 'no effect' of a treatment.
- CMV infected babies have the same average birthweight as non-infected babies;
- ß-thalassaemia does not have any effect on ferritin level;
- Recombinant human erythropoietin has no effect on the haemoglobin levels in premature infants.
Even if our hypothesis is not to do with 'no difference' it is still convention that the hypothesis being tested is known as the null hypothesis.