Attack Mechanics

Overview

The master hit chance formula is:

HitChance = Clamp( AccMods × Clamp( BaseHitChance + ToHitMods – DefMods ) )

where

• BaseHitChance is what the attack's chance of hitting would be if there were no modifiers at all – no Enhancements, no buffs, no level differences, nothing
• ToHitMods is the sum of all ToHit buffs and debuffs (the debuffs will be negative)
• DefMods is the sum of all relevant Defense buffs and debuffs (ditto)
• AccMods is the product of all Accuracy multipliers, and
• Clamp(x) limits x to the range of 5% to 95%

ToHitMods and DefMods are 0 by default. AccMods defaults to 1.

The Core Mechanics

BaseHitChance

If a player attacks a critter, BaseHitChance depends on the critter's level relative to the player's Combat Level. The critter's Rank and con color don't matter. If the player and the target are the same level, BaseHitChance is 75%. If the target is below the player's level, add 5% to BaseHitChance per level of difference. If the target is higher, BaseHitChance goes down by roughly 9% per level of difference. It's not exactly 9% every single level, but it's close. An exact table of adjustments is at the end of this article.

If a player or critter is attacking a player, BaseHitChance is a flat 50%. Critters then get Accuracy bonuses if they're above the player's level, as well as ToHit modifiers if they're lower or significantly higher. These are covered later. Direct player-vs.-player attacks don't get these adjustments.

ToHit, Defense, and Accuracy

ToHit and Defense work the same way as each other, but in opposite directions. What they do is pretty simple: they directly adjust the chance of hitting by the amount of the modifier. That is, they would be that simple if Accuracy didn't exist, but we'll ignore that for just a moment.

Example: Let's say you're attacking something with a power that has a 65% chance of hitting. If you got a +20% ToHit buff, your chance of hitting would go up to 85% (65 + 20). If your target then got a +10% Defense buff, your chance would drop to 75% (85 – 10). If you then had your ToHit debuffed by -23%, your hit chance would drop further to 52% (75 – 23).

Accuracy is subtly different. It modifies an attack's chance of hitting by a fraction of itself instead of by a fixed amount. Accuracy changes how often an attack hits in proportional terms instead of absolute terms.

Example: Consider an unbuffed, unenhanced attack that hits 30% of the time. If it got a +50% ToHit buff, its hit chance would rise to 80% (30 + 50). If it got a +50% Accuracy bonus instead, its hit chance would only rise to 45% (30 + (50% of 30), which can also be written 30 × 1.5).

Accuracy is applied in a second step, after all ToHit and Defense modifiers are applied.

Calculating Accuracy for Players

If the attacker is a player, he gets Accuracy multipliers from two places. Multiply them together to get the final Accuracy multiplier for the attack.

First, every power has an inherent Accuracy multiplier built into it. This value is 1.0 for most powers, since most powers are intended to hit a normal percentage of the time. Powers that the devs intend to hit more or less often than normal have values above or below 1.0, respectively. Inherent Accuracy usually stays within the range of 1.2 to 0.8 .

Second, a player can get an Accuracy multiplier from Enhancements. Accuracy Enhancements (but not ToHit Buff!) in the power itself are the primary source for this kind of Accuracy. Invention Origin Enhancements Sets that give global Accuracy bonuses can also add small amounts to every power. This multiplier starts at 1.0, and Enhancements can only improve it.

So for players, AccMods is calculated using:

AccMods  =  the power's inherent Accuracy  ×  (1.0 + the power's Accuracy Enhancements + all global Set Accuracy bonuses)

Calcuating Accuracy for Pets and PvE Enemies

If the attacker is a critter, its Accuracy comes from three factors. First, critter powers each have their own inherent Accuracy modifiers, just like player powers do. Second, critters get an Accuracy multiplier based on the critter's Rank. Third, if the critter is attacking a player, it gets an Accuracy (and possibly a ToHit) modifier if it's not the same level.

So for critters, we have a slightly longer formula for figuring AccMods:

AccMods  =  the power's inherent Accuracy  ×  the Accuracy of the enemy's Rank  ×  Accuracy factor due to level difference

Caps and Floors

Final hit chances are forced to stay in the range of 5%–95% to preserve a measure of uncertainty and prevent modifiers from making a power automatically hit or miss. This range limit is also applied once in the middle of the calculation, after adjusting for ToHit and Defense modifiers but prior to applying Accuracy, to prevent situations where strong Defense buffs and/or ToHit debuffs render Accuracy irrelevant no matter how high it was.

Complications

Beginner's Luck

Beginner's Luck is the community's name for a scaling ToHit bonus for low-level characters that was added as part of Issue 12. Under Beginner's Luck, characters at Combat Level 1 get a +15% ToHit bonus — exactly enough to raise their chance of hitting equal-level PvE enemies to 90% from the standard 75%. This bonus steadily decreases as the character gains levels and eventually disappears after Combat Level 19. The full table of Beginner's Luck bonuses, by level, is at the end of this article. It is almost, but not quite, +1% per level under 20.

The Beginner's Luck bonus also applies to player pets and pseudopets, though it's based on their levels, not the player's. Also, since Beginner's Luck is based on Combat Level, it is affected by Sidekicking and Exemplaring.

The Streak Breaker

There is a special feature in the game called the streak breaker. Its purpose is to prevent long streaks of misses, and it works by sometimes forcing an attack to hit automatically. Whether the streak breaker comes into play depends on an attack's expected chance of hitting, how many consecutive misses have happened so far, and what their hit chances were.

This is a direct quote from Cryptic developer Weirdbeard explaining the details. (Just to be clear, when he says "to-hit" here, he means the final chance of hitting.)

Final to-hit : misses allowed

>.9 : 1
.8-.9 : 2
.6-.8 : 3
.4-.6 : 4
.3-.4 : 6
.2-.3 : 8
0 -.2 : 100
Auto-hit powers are not included in the system.
Critters get the benefits of the system as well.
The system does not track each power individually; instead it tracks every miss you make in a row, regardless of power (or target). Otherwise you could have nine different powers, each with a 0.95 to-hit, and if you executed them all in a row you could miss each attack (note a caveat at the bottom of the post regarding this).
[AoE] attacks are considered distinct sequential attacks on indivudual targets for the purpose of the system (so if you [AoE'ed] two targets and had 0.95 to-hit for both, you be guaranteed to hit one of them).

To determine the to-hit used in the table above, you take either the current to-hit, or the worst to-hit in your current miss series, whichever is lower.

Elusivity and PvP

With Issue 13, a new feature called Elusivity was added to improve the protective strength of some Defense-boosting powers in Player vs Player combat. Elusivity bonuses were added to several Defense-buffing powers, particularly those in the defensive powersets of Scrappers, Tankers, Stalkers, and Brutes.

Elusivity is a PvP-only mechanic. Powers that grant Elusivity bonuses only do so in PvP zones: Bloody Bay, Siren's Call, Warburg, Recluse's Victory, and the Arena. Other zones don't trigger the bonus. Also, Elusivity is a player-only mechanic. No critters anywhere were given Elusivity bonuses.

Elusivity works like the opposite of Accuracy. It reduces the chance of hitting by a proportional amount, unlike Defense, which reduces it by a fixed amount. Elusivity adds an extra factor to the hit chance formula:

HitChance = Clamp( AccMods × (1 - TargetElusivity) × Clamp( BaseHitChance + ToHitMods – DefMods ) )

Not all Elusivity bonuses apply versus all attacks. Each bonus is marked with the types of attacks it works against. This system is identical to the one used by Defense bonuses, which is complex enough to deserve its own article.

Main Article: Defense

Pet Accuracy

There are two completely unrelated issues with the accuracy of summoned pets and pseudopet-based powers. One is a misconception, the other may be a bug or a design oversight.

First, many powers that work through pets or pseudopets are listed as having an unusually high inherent Accuracy, like 2.0. This is misleading, since it implies that the pet is extremely accurate. In fact, this stat is meaningless, since the act of creating a pet doesn't care about Accuracy. What matter are the Accuracies of the pet's powers, which are stored separately.

Second, player-summoned pets seem to be treated as players rather than critters for parts of the attack formula. Their base chance to hit anything seems to be 75%, with no bonus Accuracy due to their rank. Previously, it was believed that a pet's base chance of hitting things was the same 50% that all critters have and that pets got a rank Accuracy multiplier of 1.5. Both these arrangements produce the same 75% final hit chance versus equal-level enemies with no Defense, but the way player pets apparently work makes them much better than they should be at hitting highly-Defensive or higher-level targets.

It is possible this second issue applies to all summoned pets, even those summoned by non-players, since it is known to apply to Malta Auto-Turrets.

Inconsistent Terminology

Since launch, both players and developers have used the term "accuracy" interchangeably to mean all the following things:

1. actual Accuracy, as described here
2. ToHit
3. final hit chance

In fact, nearly all powers and Inspirations that provide ToHit bonuses or penalties are incorrectly labeled as "+Acc" or "–Acc". Only Accuracy Enhancements, global IO Set bonuses, Ancillary/Patron Pool power Focused Accuracy, and the Combat Training: Accuracy power available to Arachnos Soldiers and Arachnos Widows actually adjust Accuracy. All other powers and effects modify ToHit instead, regardless of game text.

Results, Notes, and Observations

• The more Accuracy an attack has, the more every single ToHit and Defense modifier affects its chance of hitting.
• A ToHit bonus of X gives you a better increase to your chance of hitting than an Accuracy bonus of X will. This is especially true against hard-to-hit targets.
• Any attack with more Accuracy bonuses than penalties will have a guaranteed chance of hitting greater than the well-publicized 5% minimum. To be exact, it'll be 5% × AccMods.
• ToHit and Defense are exact opposites. That is, increasing either produces the exact same change in the overall chance of hitting as decreasing the other by the same amount. Keep in mind, though, that a power that tries to debuff some aspect of an enemy by X will only debuff it by part of X if that enemy is resistant to that kind of debuff. So, for example, a debuff that gives an enemy -20% ToHit won't always offer the same protection as a buff that gives a teammate +20% Defense, because not all enemies will suffer the full -20.
• Accuracy and Elusivity are not exact opposites. It takes more than X points of Accuracy to compensate for X points of Elusivity — a little more if X is small, a lot more if X is large.
• Single-target control powers, interruptible attacks, and many AoE attacks (particularly Nukes) tend to have better-than-normal inherent Accuracy.
• AoE control powers tend to have worse-than-normal inherent Accuracy.
• The Combat Attributes monitor for your ToHit value assumes you're fighting equal-level PvE foes. It doesn't take into account the different BaseHitChance values for fighting different-level enemies or PvP targets.

Data Tables

Level-Based BaseHitChance
for Players Attacking Critters

Critter's Relative Level	BaseHitChance
–4	95%
–3	90%
–2	85%
–1	80%
0	75%
+1	65%
+2	56%
+3	48%
+4	39%
+5	30%
+6	20%
+7	8%

Beginner's Luck Bonuses

Character's Level	ToHit Bonus		Character's Level	ToHit Bonus
1	+15%		11	+7%
2	+14%		12	+6%
3	+13%		13	+5%
4	+12%		14	+4%
5	+11%		15	+3%
6	+11%		16	+3%
7	+10%		17	+2%
8	+9%		18	+1%
9	+8%		19	+1%
10	+7%		20+	+0%

Level-Based Accuracy and ToHit Modifiers
for Critters Attacking Players

Critter's Relative Level	LevelDifferenceAcc	ToHit Modifier
–3	1.0	-27%
–2	1.0	-19%
–1	1.0	-10%
0	1.0	0
+1	1.1	0
+2	1.2	0
+3	1.3	0
+4	1.4	0
+5	1.5	0
+6	1.5	+5%
+7	1.5	+10%
+8	1.5	+15%
+9	1.5	+20%
+10	1.5	+40%

Rank-Based Accuracy Multipliers for Critters Attacking Anything

Critter Rank	RankAcc
Minion, player Pet	1.00
Lieutenant	1.15
Boss, Elite Boss, Sniper	1.30
Monster, Giant Monster, AV	1.50

References

• Arcana's Guide to Defense v1.4 - Updated for I7
• Re: Guide to (Tohit and) Defense v1.10beta
• Elusivity