The hypothesis which is to be tested is called the null hypothesis. It is denoted by . It is a starting point in the investigation, and a statement which we hope will be rejected is taken as a hypothesis. The modern approach is different: Today any hypothesis we wish to test is called the null hypothesis and is denoted by . However in this post we shall follow the old convention, and any hypothesis will be called the null hypothesis only when we hope to reject it. Thus the null hypothesis is framed for possible rejection.
For example, take the hypothesis tall fathers have tall children. We shall assume that tall fathers do not have tall children. This will be considered as the null hypothesis and will be denoted by . We are hoping that will be rejected on the basis of sample data. Take another example: drinking coffee increases the chances of heart attack. To start with we shall assume that heart attack has no link with drinking coffee, so this will be taken as and we hope it will be rejected by the sample data.