(a) Apples grow to different sizes, from small to average and large. Farmers want large apples.
(b) There is this rare bug, the apple bark bug, which lives in the bark of apple trees. Due to its rarity, the apple bark bug is not well understood.
(c) Agronomists are divided about the effect the apple bark bug has on the size of apples. Some believe that apples from trees that have been infested with colonies of this bug are smaller than on trees without such an infestation; others believe that in fact they are larger.
The ministry for agriculture has commissioned a study on the effects of the apple bark bug on apple size. Based on the results of this study they want to either exterminate the apple bark bug, ignore it, or cultivate it. There is no scientific literature on the matter, there is no reliable data, and the research institute commissioned with this study now wonders:
What should be the null hypothesis?
As natural scientists they know they cannot present their limited observations as general fact, but only falsify a hypothesis. But what should it be?
The third semester student apprentice from the nearby university, who has just finished a course in the theory of measurement, suggests:
H0 : Applebug = Appleno bug
H1 : Applebug ≠ Appleno bug
His friend, who has heard the same lecture, rolls her eyes and suggests:
H0 : Applebug ≤ Appleno bug
H1 : Applebug > Appleno bug
The research group leader smiles and compliments them on their attentiveness.
What does he suggest?
This is, of course, an example, illustrating a question relating to psychological research: How do we define the null hypothesis, if we have no presupposition about the effects of an intervention?