null hypothesis :

The possibility that there is no relationship between the variables in a statistical analysis.

In any statistical investigation, the researcher is looking for relationships or differences. For instance, is there a relationship between smoking and lung cancer? Are people who take vitamin supplements statistically healthier than the general population?

In all of these situations, the possibility that is being investigated is the research hypothesis. For instance, "there is a relationship between smoking and lung cancer" is a research hypothesis. It's a statement that might or might not be true (although is now considered to be true). The alternative possibility, that the researcher is wrong and there is no relationship,