A worker on a fish-farm notices that his trout seem have more fish lice in the summer, when the water levels are low, and wants to find out why. His research leads him to believe that the amount of oxygen is the reason – fish that are oxygen stressed tend to be more susceptible to disease and parasites.
He proposes a general hypothesis.
“Water levels affect the amount of lice suffered by rainbow trout.”
This is a good general hypothesis, but it gives no guide to how to design the research or experiment. The hypothesis must be refined to give a little direction.
“Rainbow trout suffer more lice when water levels are low.”
Now there is some directionality, but the hypothesis is not really testable, so the final stage is to design an experiment around which research can be designed, a testable hypothesis.
“Rainbow trout suffer more lice in low water conditions because there is less oxygen in the water.”
This is a testable hypothesis – he has established variables, and by measuring the amount of oxygen in the water, eliminating other controlled variables, such as temperature, he can see if there is a correlation against the number of lice on the fish.
This is an example of how a gradual focusing of research helps to define how to write a hypothesis.