The following is an excerpt from page 14 of the Warhammer Annual 2002 release with an expanded explanation by Gav Thorpe regarding Combat Resolution and Outnumbering.

Weight of numbers counts for a lot in Warhammer these days, and the +1 Outnumbering bonus is one of the most important ways to ensure that characters retain a healthy respect for large blocks of enemy soldiers. We've had a few questions regarding the Unit Strength of certain troop types, so on page 135 is an expanded version of the Unit Strength table. Unit Strength is not just a measure of numbers though, it also takes into account such factors as the size of the warriors, how fearsome they are, etc. Having a Goblin charge you in the flank won't scare you much, but having a massive Dragon pouncing on you will! As we produce more Warhammer Armies books, we'll detail the Unit Strength values of particularly weird and wacky troop types, but until then you should be able to fit them into one of the categories below.

Some units may contain a mix of different troop types, and all relevant Unit Strength values are added together. For example, 3 Rat Ogres with 2 Skaven Packmasters would be 9 + 2 = 11 (3 each for the Rat Ogres, 1 each for the Packmasters). Also remember that unlike the Rank Bonus, which is counted at the start of combat, Outnumbering is worked out after casualties have been removed.