With thanks to Wisden.com, where I found the best explanation.
If two group winners finish on equal points, the following tie-breakers will come into play. In this order: most wins, fewest losses, the result between the sides in the group stage, most wickets taken, most runs scored.
Most wins (at start of final round)
Somerset, Essex (3)
Derbyshire, Worcestershire, Yorkshire, Kent, Middlesex, Hampshire (2)
Fewest losses (at start of final round)
Somerset, Worcestershire, Derbyshire, Yorkshire, Essex (0)
Kent, Middlesex, Hampshire (1)
Results between sides
Somerset v Worcs – currently being played
Kent v Hants – in play.
Essex v Middx – in play
Essex v Kent – Essex win
Middx v Hants: Hants win
Middx v Kent: draw
Essex v Hants: draw
Derbys v Yorks: draw
Most wickets (at start of final round)
Somerset: 78
Kent: 69
Worcestershire: 66
Middlesex: 67
Hampshire: 63
Essex: 60
Derbyshire: 55
Yorkshire: 52
Most runs (at start of final round)
Worcestershire: 2,156 runs
Middlesex: 1,882 runs
Somerset: 1,847 runs
Kent: 1,759 runs
Derbyshire: 1,679 runs
Essex: 1,611 runs
Yorkshire: 1,573 runs
Hampshire: 1,058
