Just like a visit to the doctors, we assess a test site by comparing its biota to what we would expect if it was in reference condition (i.e. doing ok). We then try to determine how human activity is affecting the biota in the ecosystem. In this chapter, I use the two example datasets to illustrate my method of using the Reference Condition model to assess how closely each Test site aligns with our expectations if the ecosystem was "in good shape". I then build Diagnosis models to determine exactly how, and how much, human activity is affecting the biota in the exosystem.