I've no idea about exact figures, but in my experience he shows up far more often, hangs around longer and hunts more aggressively on the higher difficulties. On easy I got through entire chunks of the game without ever seeing him, but on hard he would crop up all the time.
Is there any data on how the difficulty level affects the Alien? For example, on Normal he can see the player from 20 meters, on Easy - 10 meters, on Hard - 30 meters, and so on.
I'd like to know what actually changes with the difficulty, as I don't see much difference playing on Normal after I beat the game on Nightmare.