Null Hypothesis

In research, there is a convention that the hypothesis is written in two forms, the null hypothesis and the alternative hypothesis (called the experimental hypothesis when the method of investigation is an experiment).  Briefly, the hypotheses can be expressed in the following ways:

o The null hypothesis states that there is no relationship between the two variables being studied (one variable does not affect the other).

o The alternative hypothesis states that there is a relationship between the two variables being studied (one variable has an effect on the other).

The alternative hypothesis prediction is one that states that results are not due to chance and that they are significant in terms of supporting the idea being investigated.  A null hypothesis prediction is one that states results are due to chance and are not significant in terms of supporting the idea being investigated.

In order to write the null hypothesis for an investigation, you need to identify the key variables in the study.  A variable is anything that can change or be changed, i.e. anything which can vary. Examples of variables are intelligence, gender, memory, ability, time etc.

Let’s consider a hypothesis that many teachers might subscribe to: that students work better on Monday morning than they do on a Friday afternoon (IV=Day, DV=Standard of work).  Now, if we decide to study this by giving the same group of students a lesson on a Monday morning and on a Friday afternoon and then measuring their immediate recall on the material covered in each session we would end up with the following:

o The experimental hypothesis states that students will recall significantly more information on a Monday morning than on a Friday afternoon.

o The null hypothesis states that these will be no significant difference in the amount recalled on a Monday morning compared to a Friday afternoon. Any difference will be due to chance or confounding factors.

The null hypothesis is, therefore, the opposite of the experimental hypothesis in that it states that there will be no change in behavior.

At this point you might be asking why we seem so interested in the null hypothesis.  Surely the alternative (or experimental) hypothesis is more important?

Well, yes it is. However, we can never prove the alternative hypothesis. What we do instead is see if we can disprove, or reject, the null hypothesis. If we can’t reject the null hypothesis, this doesn’t really mean that our alternative hypothesis is correct – but is does provide support for the alternative / experimental hypothesis.

How to cite this article: McLeod, S. A. (2011. Simply Psychology; . Retrieved from