If you say that a particular activity is not anexact science, you mean that there are no set rules to follow or it does not produce very accurate results.
Forecasting floods is not an exact science.
exact science in British English
(ɪɡˈzækt ˈsaɪəns)
noun
1.
any scientific field in which accurate quantitive techniques are used and there are accurate means of testing hypotheses and repeating results