I think a 15 minute interval is just enough to qualify - after all,
given that the average wait should be 7.5mins, that's just enough
time
at the big terminals to buy a ticket and find the platform if
you're
unfamilar with the place.

I think worst-case time is more important than average - i want to
know
i'm not going to have to wait longer than a given time.

In Putney theres a sign advertising up to 6 trains per hour (off peak)
to Waterloo. Thats fine except on a Sunday when all 6 trains leave
between ten to and twenty past