Thursday, January 09, 2014

Rewriting the attack roll

I have an issue with skills and combat resolution. Currently skill is rolled one way and combat is rolled another way, when in truth combat is no other thing than the application of combat related skills. So why do players have to beat a high value with skills and low value in combat?

Well, in the early stages of the game it was intuitive to handle low roll resolution. If you needed a delta 10 to hit (see delta below) and the enemy has 80% cover that converts to 2 or less to hit (10 - 8 [80% of 10]). Please note that a roll above 2 and below 10 still hits the enemy's cover and may still injure the target if the shot successfully penetrates the cover, or does so automatically if the enemy enjoys only 80% concealment, not cover.

Now, I see a couple of issues with this. First of all skill has evolved to be a roll high die roll. As a player you need to overcome a certain target value, say 35 for really hard tasks, 25 for difficult tasks, 15 for easy tasks 5 for trivial and so forth. As a unit, players may stack skill and attribute bonuses from all characters involved in the task resolution. So a delta 28 (quite hard) may be brought down by subtracting the skill bonus of the two or three team members working on the task plus any applicable attribute bonuses. The required value may then end up around 18 or 15, much, much easier to do.

So as a player you're doing all these roll high checks and all of a sudden combat gets turned around and now you have to roll low. This can be a little confusing. Another issue is that in the worst case scenario you have a 3% chance of hitting (which is the odds for a delta 0). In my opinion that's sometimes too high and a bit unrealistic. Some attacks have a much lower chance of hitting than that. PS, as a side note Weapons Free considers suppression fire, so shooting to kill isn't always a goal.

As a GM you don't want to grant the players the chance to wipe out all the opposing force in one strafe, nor grant the enemy the same against the party. This high chance of hitting with a low value also makes it hard to distinguish the skilled from the unskilled. In other words the harder the shot the more susceptible the delta is to modifiers. So a hard long distance shot goes from 3% (delta 0) chance to 25% chance of success (delta 3) with a simple + 3. What's the point of being a sniper then when your average Joe Shooter can get a few pluses in a mile long shot?

Now, if I turn it around and have combat resolved the same way as skill then high rolls will be required for more difficult shots instead of lower rolls. A 20 or higher instead of a 2 or lower. The issue arises with cover, suppression and other combat related elements which are currently handled in a roll low manner. For example if a shot requires 20 or higher, how does cover affect this? With the roll low it was simple, 50% cover converts the 20 to 10 (half the value). Now, with the roll high, 50% is somewhere between 20 and 38. Subtract 20 from 38 divide by two, add to 20.... math is getting complicated and we don't want that.

So maybe I should just dump the whole math/percentile thing and put tags on different values: impossible, epic, very hard, etc. and let the GM sort it out. So a normal shot at a very long distance may be impossible for the unskilled shooter. With some marksmanship it becomes epic, with sniper training it drops to very hard and if your character happens to be Vasily Zaytsev reborn it drops to easy. This "humanized" terminology is better to grasp as a player and GM and to adjust intuitively than a percentage rule applied to a number.


Delta refers to the difference between two 2d20 rolls. The player and GM both roll and the difference between them is the delta value. To roll below a given target delta is to have a delta below the given value. To roll above means to have a delta above the given value.

The 2d20 - 2d20 probability distribution is as follows (graph shows probability of failure when a certain value is required):

The higher the required delta value the less probable the success. Needing a 20 or more implies a 90% chance of failure while 10 or more only a 65% chance of failure.

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