Evaders must make a DEX roll; pursuers make a WIS roll (I allow for different abilities if circumstances are weird and the player makes a case for it). Smaller groups have the advantage when evading or pursuing: take the difference between the two groups' sizes and divide by 5, then apply that number as a bonus to the smaller side's roll. Evaders get a +1 bonus for having a thief, natural cover, darkness, precipitation. Pursuers get a +1 bonus for having a ranger, pursuing on mud or snow, long-range vision.
If the evaders win, they slip away; if the pursuers win, they pursue.
During a pursuit, each side makes a DEX roll each round. The faster party gets a +1 for every 3" of movement over the slower group (e.g., Move 12" being chased by Move 9" gives the pursued a +1 bonus). Check results for each side below:
* Critical Failure: pursued or pursuer loses control, crashes, gets injured, or otherwise cannot continue.
* Failed: pursued or pursuer gets free attack; chase continues.
* Success: pursued or pursuer gets +1 bonus on next round's roll.
* Critical Success: pursued gets away or pursuer catches his prey.
If both sides get the same result, the round is a stalemate (regardless of that result).
For example, round 1 the pursued fails the DEX roll, but the pursuer succeeds, so the pursuer gets a free attack and a +1 to next round's roll. Round 2 the pursued gets a critical success while the pursuer succeeds, so the pursued get away. Done.