Hi there
When looking at collisions, or near collisions, in the modern, largely TCAS II equipped era, we must also remember that routes are now being flown through far more precise and exacting corridors of airspace using GPS and IRS navigation systems, and these routes are being flown through some very poorly manned and ill equipped airspace (i.e. Africa) therefore increased chances of colliding with other aircraft is higher.
I am of the opinion that when looking at post-TCAS vs pre-TCAS collisions, we need to take that into account as it does not give an accurate portrayal of its effectiveness.
Cheers
Anthony