I just watched The Sign of Three, where several women claim to have dated a ghost. At some point, Sherlock narrows his questioning down to four women.
What was his criteria to choose these four women in particular?
No explicit explanation is given.
Recall that he's not actually interviewing these women in person, the "filtering" is actually taking place in his mind.
He's actually reviewing stories posted on the fictitious idatedaghost.com.
Thereafter, he "speaks" with the selected few on message boards / forums to get more details and compare notes...