Damage Threshold

Damage Threshold is a combat statistic in Fallout, Fallout 2, Fallout Tactics: Brotherhood of Steel, and Fallout: New Vegas. It was removed in Fallout 3, leaving Damage Resistance as the only armor statistic. However, it was restored in Fallout: New Vegas, before being removed yet again in Fallout 4.

Fallout, Fallout 2, Fallout Tactics
In the original Fallout games, DT (Damage Threshold) is one of three stats by which a character can reduce or avoid damage. The other stats are AC (Armor Class) and DR (Damage Resistance). DT occupies the "middle step" for combat and simulates the effect of how tough armor can immediately stop a bullet cold, no matter how many successful shots are fired (like a real-life tank).

More specifically, after AC is checked for a successful hit and before DR is checked to further reduce the damage, any incoming damage is immediately reduced by DT:
 * $$Adjusted = \text{max}(0,\ Base - DT)$$

Unlike in Fallout: New Vegas, damage can be completely negated by DT. In such a case, the player gets a message that an attack dealt "no damage," which can be frustratingly common when using weaker weapons against power armor or Wanamingos.

Mechanics
Fallout: New Vegas mostly moves away from the exclusive use of Damage Resistance (DR) in Fallout 3, towards Damage Threshold (DT). Like in previous games, DT can function as an outright subtraction: if a shot is fired with a damage value of 40 against a DT of 10, then 10 damage will be ignored.

However, the sequence of calculations differs compared to previous games and makes the math a little bit more involved than in Fallouts 1 and 2. Thus, no discussion of damage is complete without referring to the combined statistics of DR and, after it, DT. This is because, contrary to previous games, DR is applied first. The value after this adjustment depends on any DR a player has, which is capped to a DR of 85 (85%); however, DR above 80 does nothing, so the 85 cap might as well be an 80 cap.
 * $$Damage_{DR}=Basic Damage \times \frac{100-\text{min}(DR, 85)}{100}$$

DT applies after that, with a limit: final damage taken cannot be lower than 20% of the basic attack (a term equal to "dam" argument given in Combat article; see the section); if lower, it is overwritten to 20% instead. This means that the higher your DT, the lower your functional DR cap, depending on the damage you are trying to resist, and vice versa. For example, with DR 80, DT above 0 won't do anything, regardless of the incoming damage; however, with e.g. DR 70 against an attack of damage 10, a DT of 1 will reduce damage taken from 3 to 2, and DT above that will not help. Provided your DR is 79 or less, any amount of DT is theoretically useful, and it will have a greater impact the lower the incoming damage. Applying our previous example to an incoming attack of 100 instead of 10, DR 70 DT 1 would reduce damage instead from 30 to 29 - a much less significant reduction than 3 to 2.


 * $$Damage_{Final}={max}(Damage_{DR}-DT, Basic Damage \times 0.2)$$

So, to evaluate our earlier example, if a shot of damage 40 is fired against a character with 95 DR as well as the previous 10 DT, the damage is first reduced to 6 (95 DR is capped down to 85% reduction, 40 × 0.15 = 6), and then reduced by 10. The final result (-4) is less than 20% of the base attack (8), so the damage is raised to 8 instead. This example shows that DR of 81%+ is redundant, as the final damage can only be reduced by up to 80%.

Damage which is reduced to below the 20% minimum value is indicated with a shield icon next to the health bar (either the PC, or the non-player character's). When all of an enemy's DT (if it's not 0 to begin with) is overcome with armor piercing attacks, then a broken shield icon will appear in its place instead. The shield icon is shown during V.A.T.S. and on the HUD's health bars once damage has been inflicted upon a target and will remain for 3 seconds afterwards.

The default value for DT is set at 0; this is then modified mainly by the DT value of armor and clothing worn. Other attributes can affect the DT value of a character, both permanently or temporarily. Perks and consumables can increase the player's DT, while the Kamikaze trait reduces it. When attacking, the use of ammunition can modify the target's DT either way, with Hollow Point generally increasing their DT, and AP decreasing it.

Difficulty and critical damage impact
As with the player's damage, the difficulty multiplier of incoming damage is applied after the damage resistance and damage thresholds calculations. As an example, take vanilla deathclaw's damage –125– on very hard difficulty, where inbound damage multiplier is ×2. If the player has no DT and DR, the damage dealt by the monster is plain 250 hp. If the player has, for example, 40 DT and no DR, the damage becomes (125 – 40) × 2 = 170; if the sequence were altered (DT subtracted after multiplier applied), the player would have received 210 damage instead. This makes DT way more vital on higher difficulties.

Unlike difficulty multiplier, the critical damage is applied before the impact of damage threshold calculations as it counts as the 'base damage'. The deathclaw mentioned above deals additional ("+") 125 damage when it crits, leading to an attack with the power of 250 on normal difficulty. With 40 DT, player suffers from 210 damage instead, leading to 420 damage on very hard difficulty.
 * While unimportant in such a case, it becomes vital when the enemy has a low power attack which is reset to 20% of the 'base damage' due to the cap mentioned in the previous section. For example, if young deathclaw's damage is 40 and Courier's DT is 40, the attack will deal 8 (40 × 0,2) damage on normal difficulty. However, when young deathclaw crits, it deals additional 40 damage resulting in 80 total damage. With player's DT applied, the final damage becomes 40 and, surpassing the 20% criteria, deals the raw damage. On very hard difficulty, it leads to 16 (non-crit) and 80 (crit) damage, respectively.

Comparison between Damage Threshold and Damage Resistance. Implications
DR factor becomes much more important in extreme cases. Vanilla deathclaw (like any other enemy) will completely ignore the DT factor if the Courier has 80%+ DR (Med-X + Slasher + Battle brew combo gives 85%) and deal 50 damage on very hard difficulty in any case, against any DT index, as the damage cannot be lower than 20% of its basic attack ((125 × 0.2 = 25) × 2 (very hard difficulty multiplier) = 50); its critical damage will be capped at 100 instead (compare to 500 raw crit damage against the Courier with no protection and 420 damage against the  courier  with 40  DT). The armor indicator will appear upon the hit as well, confirming the deathclaw is dealing it's minimum possible damage. It means that when the Courier has to deal with high damaging opponents (like said deathclaws), the most reliable method is to increase DR to its maximum wearing no armor at all for faster movement – or the armor with the best possible buffs. If the player has no Battle brew (→ 50% DR), then, against the said deathclaw, 37.5 DT is needed to cap creature's damage to its minimum (125 × 0.5 = 62.5; 62.5 – 37.5 = 25).

As a more extreme example, take Legendary bloatfly from Old World Blues add-on. Its plasma damage is set to 300 (equal to that of deathclaw alpha male), becoming plain 600 on very hard difficulty. However, with 80%+ DR it's attack power is capped at 60 at normal and 120 at very hard difficulty, making it possible to survive several blasts for a high-level Courier. With 50 DT and no DR, surviving its attacks dealing 500 damage on very hard difficulty ((300 – 50) × 2) is possible only on a very high level. With 50% DR and 50 DT, the damage becomes 200 ((300 × 0,5 – 50) × 2) on very hard difficulty. Interestingly, although no DT and DR of 50% makes very hard difficulty experience equal to no armor experience on normal difficulty, the DT factor alters the picture. If the normal difficulty deathclaw hits the player with no DR and DT of 40, it deals 85 (125 – 40) damage; however, the very high difficulty deathclaw hitting the Courier with 50% DR and 40 DT results in 50 damage instead (125 × 0,5 – 40 = 22,5; → override to 125 × 0,2 = 25; 25 × 2 = 50 (very hard multiplier applied)).

To sum up, high DT is enough against low- but fast-damaging opponents (e. g., any fast-firing humanoids) unless they don't get critical hits; however, enemies with powerful attacks (mainly deathclaws and humanoids with explosives or single-action high-end weapon) should be countered with maximum DR instead.

Bugs

 * Despite DT being the determining factor for armor in Fallout: New Vegas, when repairing armor, merchants will list DR instead. All armors when viewed from the repair screen have a DR of 0. This is fixed in patch 1.1.1.
 * In the initial release of Fallout: New Vegas, all creatures such as super mutants, robots, or deathclaws had twice as much DT as intended (in other words, twice the amount of DT as was entered in their G.E.C.K. page). This was corrected in the 1.2 patch. A similar bug affects the DR of creatures in Fallout 3, which has not been corrected.
 * In Fallout: New Vegas, the only articles of clothing that provide DR are the rebreather, which is acquired during the quest Volare!, the Vault 11 jumpsuit and the Scientist outfit.
 * While wearing one of the said items above or after having taken Med-X, Slasher, or battle brew, your Pip-Boy 3000 will crossfade between showing your DR and DT. Taking the DR items off or having your DR chems expire while your Pip-Boy has DT partially faded out or completely replaced with DR will keep your DT partially faded out or render it completely invisible. Removing the DR item or putting away your Pip-Boy only when the armor display is fully on DT is highly recommended.