enginecon

In principle, it is reasonable to consider program strength in specific fields and even subfields.

The main problems of doing so are two:
(1) the candidate may just have a "leaning" toward some field(s) -- not really a well-defined research idea --, and this may change as s/he becomes aware of new material during the 1st/2nd year of the PhD

(2) Even if the candidate has a strong field preference and some concrete ideas, s/he still need to *predict* the field strength about 3 years down the road, when s/he is likely going to start research work in earnest... such predictions are quite tricky, especially outside the top 20 and for not-so-large departments, because the departure or arrival of a single professor can have a significant impact in the "field ranking".

So, if you choose among programs based on field strength you may find that by the time you actually start your research work the field situation is quite different from what it was when you chose among programs (your interests, or the available faculty may have significantly changed since 3 years before).