Full Idea
A cause ought to increase the frequency of the effect, but this fact may not show up in the probabilities if other causes are at work.
Gist of Idea
A cause won't increase the effect frequency if other causes keep interfering
Source
Nancy Cartwright (How the Laws of Physics Lie [1983], 1.1)
Book Reference
Cartwright,Nancy: 'How the Laws of Physics Lie' [OUP 2002], p.23
A Reaction
[She cites Patrick Suppes for this one] Presumably in experimental situations you can weed out the interference, but that threatens to eliminate mere 'probability' entirely.