Tom,
I suppose the trickiest part here is to measure just what exactly is "success and failure in battle". Several aspects should probably be measured in order to have an accurate picture of how any given character behaves in war. There could be a pie chart of its own about this, actually.
On one hand you could measure simple activity by increasing the stat every time the character goes to battle. We could call that "eagerness". Then you could measure the battles in which he is on the winning side and his unit is not retreated or wiped out. That could very easily be "prudence". Then you could measure battles where the character is on the side that has somewhat lower chance for success (CS and defenses taken into account) and/or when the unit of the character is on the front lines/leads a charge. We could call that "courage". What's left after all this is victories in important battles when the odds are against you, which we could call "valor".
Further, each stat diminishes by itself over time, but valor decays very slowly, and could even be measured in absolute terms in the pie chart while the rest of them would share what's left from valor in relative terms. Eagerness would drop the fastest.
Example 1: A noble only ever goes to battles when he is sure he is on the winning side. His eagerness hardly ever rises, while his prudence gets frequent marks. Thus he's always low on courage and never gets valor.
Example 2: Another noble always rushes to war, no matter what the odds are, so he gets high marks on eagerness and on courage, and if he is successful, also some valor. But he will remain very low on prudence.
Example 3: Yet another noble seldom goes to war, but only does so when there is a great need. He will not be very eager, but can be seen as courageous and even valorous if he succeeds.
Valor would be the most important statistics in determining what kind of troops you will be able to recruit. Only the top 20% most valorous nobles in a realm can recruit special forces. The top 30% could recruit cavalry and top 50% could recruit the best available standard units.
Prudence, on the other hand, would effect the morale of your troops. The more prudence you are reputed to have, the more the troops trust you not to get them killed. The less prudence you have, relatively speaking, the more your troops are likely to desert you.
Courage would gain you respect in the eyes of your troops, and would let you get away with lesser payments.
Eagerness on the other hand would determine how many you could recruit. The soldiers want to see battle.
