Complex sentence: They rely heavily on the number of cases where the two events occur together: in this example, instances of both pain and bad weather but pay relatively little attention to the other kinds of observation (of no pain and/or good weather).
Split into simpler sentences: They rely heavily on the number of cases where the two go together: in this example, instances of both pain and bad weather. They pay relatively little attention to the other kinds of observation (of no pain and/or good weather).