I was reading Bio-statistics principles and practice by Antonisamy and stumbled upon the following: Null hypothesis is a hypothesis that suggests an absence of difference, association or effect, the negation of which provides evidence for presence of difference, association or effect. The only problem I'm facing with this definition is that it doesn't say- differences between what, association between what and effect of what on what? From random sources I know that a null hypothesis involve hypothesising that a theoretical distribution is consistent with an observed distribution, i.e. there is no difference. Also it can assume that there's no difference between the scores of two variables. But I don't clearly understand what the book meant by association and effect? I actually found this post in a different community where the OP is trying to establish a relationship (association) between driving speed and gender. Does the definition refer to such associations?