The hypotheses need to be carefully formulated since a goal in experimentation is to accumulate results which allow the researcher to falsify the null hypothesis so that it can be rejected with some degree of certainty and the alternative hypothesis can be accepted. For instance, an observation that the parasite behaves erratically when in the vicinity of a host might suggest to you that chemical metabolites of the host are sensed by the parasite.
Parasites use chemoreception in host location.
For example, we might deduce that if a parasite were in the prescence of host chemicals, then its behavioral orientation with respect to the chemicals would be random if H o is true. Such a deduction can be easily evaluated experimentally using a simple protocol in which the parasite is given the opportunity to either swim to a site containing the host chemical or to a site without host chemical. This type of approach to experimentation is called the hypothetico-deductive method and is very useful in science. If H o is correct, we would predict that the parasite would enter the chemical-containing arm only half of the time. These numbers are the observed frequencies.