a) Criteria should be as detailed and objective as possible. So, the 'extent' of something is rather vague and should be avoided. The other three are objective quantitative measures and are preferred. However, the average subsidence, does not give information related to the decline of nature. A large subsidence taking place locally and a small subsidence taking place globally, can have completely different results. Thus, this criterion is also disregarded.
b) Other criteria might be: