The John Sullivan of interest in Pittsburgh was a John L. who was a policeman. I found him easily in a 1928 directory. In looking at all the John Sullivan entries, I located a John J. who was also a policeman. This is something I need to be aware of. Individuals can easily be listed in records without their initials and it would be easy to inadvertently confuse these two people.
Even if you find “your person,” browse other names. In addition to potentially locate others living at the same address (at least in directories), you may learn of others whose names are very close to your ancestor with other similar characteristics that may cause them to be confused.