'In inferential statistics, the term "null hypothesis" is a general statement or default position that there is no relationship between two measured phenomena, or no association among groups.' (wiki) The book I'm following has to say : Q: In a nutritional study 13 students were given a usual diet with vitamin tablets and 12 set of other students were given only the normal diet. After 12 months their weights are measured as given below (a 2 x 13 table in which weight gains of the two set of students are mentioned) Can you say that vitamins were responsible for this difference? A: Ho= Vitamins are responsible for this difference HA= just the ooposite, not responsible The t-value turned out to be more the p-value and they rejected the Ho P.S. I'm sure it isn't right because Vitamin rich diet does cause weight gain and given the concept of null hypothesis the Ho assumed in the excerpt should be wrong.