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Background: Our group does D&D 3.5/4e one week, and a different system the other week (we've done GURPS and Serenity, and I'm going to be running Mutants and Masterminds 3rd edition). I've (almost) always GMed the "other" week.

Something I've noticed is during D&D 4e combat, the defender doesn't have any impact in the battle. The attacker rolls "to-hit", then if he matches or exceeds AC, the attacker rolls damage and applies conditions. In GURPS and Serenity, the defender is involved in the battle -- either the defender has to roll a dodge / parry separately for the attack to hit, or it's a straight contested roll against a defense of the defender's choice.

This seems to have impacted the games:

  • The flow for 4th edition seems to be faster. When it's player A's turn, everyone focuses on that person. In the other game, there's a lot of exchange between player and GM during combat (especially GURPS, with the "OK, I hit, now what type of defense do you want.").
  • When the GM is acting in 4th edition, we know that we're not going to be involved. We're paying attention, but it's more like watching TV; we're not involved as much.
  • In GURPS, a player rolls an attack really well, and then if I as a GM get a great dodge roll, that attack roll is nullified. Player is sad.
  • Players can spend bennies on their defense roll -- this reduces the drama a bit on combat.

So, I was looking at the Mutants and Masterminds rules. Like D&D 4th edition and other d20 games (and unlike other games), M&M doesn't have a choice for the defender during combat. The basic attack system is "attacker rolls an attack, then the defender rolls toughness, with the target value being attacker's damage + 15." I was thinking that that can be simplified by "attacker rolls an attack, then rolls damage against the target's toughness rank (toughness + 5)." But, that essentially makes the defender not be involved at all in the attack process -- no dice rolls, nothing. I'm not sure I like that. But, does it matter in the long run?

TL-DR: How does the "flow" of combat change between a system like D&D 4e (where the attacker rolls all the dice to determine the outcome) and GURPS (where the defender has the chance to defend)? If I house-rule a game system from one to the the other, is there anything I need to worry about? Thanks.

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4 Answers

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The short of it is that, mathematically:

  • Whether attacker or defender rolls against a flat target makes no difference. None.

  • An opposed roll changes the probability distribution, making it more swingy.

About the second point: In d20 you normally replace the target of 10 + mods with 1d20 + mods. So you had a 50% chance of succeeding before hand, that's basically unchanged. If you had a very small chance of succeeding, now you have better odds. If you had a large chance of succeeding, now you have worse odds.

You can see that in this anydice output, which represents the two d20 methods. Click on view:graph and data:at least to best contrast the two distributions. Note how if you needed a high number on your original d20 roll, now you have a significantly better chance. And with opposed rolls, it's possible to succeed or fail on checks which would be automatic failure/success on a single die.

Now, these different options can definitely change the feel and flow of the game, but those things are pretty subjective. So I'm limiting my answer just to the math.

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One complaint about systems where both attackers and defenders use opposed rolls is that it can get very swingy and unpredictable. I'm not well-versed enough about GURPs, but you may also want to look at systems where you as a defender can potentially have an influence on the flow of combat.

  • Qin has both active and passive defence. Players get a finte number of actions per turn. If they choose to, they can spend an action to roll the dice and their defense score to it. If they don't, the enemy just have to hit the defender's passive defense.

  • RuneQuest 6 allows the defender to make a Parry roll to deflect an attack, which costs an action like Qin. If you roll well as a defender, you can inflict combat effects on the opponent.

Combat systems like those present a tactical choice between passive defense and active defense, but as always, with more options means that the combat round could be longer.

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GURPS sounds similar, except that the defender always rolls something; you can choose to Dodge (which is unlikely) or you can Parry (which is more likely, but can only be done once a turn). M&M is nice in that it allows you to make tactical choices during your attack phase that affects your defense (e.g. all-out attack subtracts your defense), but it's only during your turn. –  bryanjonker Jul 29 '13 at 17:15
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Really, depending on how well the designers do their math, the mechanical difference between rolling and not in the long term can be insignificant, though obviously the chance to have more extreme outcome sets is somewhat notable.

When you have an opposed test, you get a much larger curve, obviously, since a 1+/- something is different than a 1, and a 20+/- something is different than a 20.

As far as game flow, anything that involves an opposed test is going to require a little more bookkeeping (even if it's just checking a value), and take a little more time.

However, this doesn't necessarily result in huge mechanical differences; a d20 system built one way versus a d20 system built the other way can still function identically in terms of success/fail outcomes; you may decide to apply a modifier, for instance, to Armor Class if you convert a GURPS character over (not that I'm really an expert on GURPS), or vice versa with GURPS' appropriate defensive abilities.

So, in short, no, converting systems just requires you to do a little math to get the exact same success/fail rate (of course, if you're not dealing with simple success/fail in cases of degrees of success/failure, that complicates things a little, but you can still do the math).

I've known people who make characters roll their AC with 10 as the "no modifier value" in D&D games, and while it occasionally allows for some kinda stupid hits (i.e. defender rolls a 1 against a kobold whose full attack value, after modifiers, came to something like 3), it doesn't break the game or anything.

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Tunnels and Trolls has a system that we have adopted for our homegrown system. There is no attacker and defender as such.

Both combatants roll and whoever rolls highest has hit the other. The amount of damage is determined by the difference.

This allows fast, interactive combat and more directly mirror our experience of combat (we are all medieval sword fighters, most of us instructors) where neither party really chooses to defend and do nothing else (apart from very specific situations).

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