18 is a pyshcological barrier of sorts, but it is also what the technicians call 'resistance'...Once broken through decisively ( it broke once last year to 18.3...) it often becomes what they call 'support'...much like the 16.5 you noticed is....

I don't have any more clue where it'll be in 12 months than I know how to build a spaceship...

"...the left considers you vermin, and they'll kill you given the chance..." Bristoe