How Stealth and Detection really works  Proposal, as difficult to confirm all aspects. Try negative Search to auto fail detecting traps; a snail can detect a lot of traps. Also tried to counter the +5 DC to Detect Mode during Stealth but then when Searching for Traps, I have no other explanation.

Stanley Woo's take on Stealth and Detection is so widespread that it may be considered canon. The Passive 5ft detection is as the Game Manual suggests, but 5ft is also the same distance for Attacks of Opportunity and even Touch range is 7ft. This is would be very close for detecting traps and short of what can be achieved. In comparison 5 meters in game is half way into a small room or 4 paces.

Passive Search (walking) 
Trap detection radius: 5m (half tile; line of sight)
Trap detection roll: d20 (half skill running)
Trap detection check: briefly every 3 seconds
Spot or Listen detection roll: d20 (half skill running)
Spot or Listen detection check: briefly every 3 seconds
Detect Mode (or Keen Senses)
Trap detection radius: 10m (one tile; 8 paces)
Trap detection roll: d20
Trap detection check: briefly every 3 seconds
Spot or Listen detection roll: d20
Spot or Listen detection check: always active and reroll every 3 seconds
Stealth
Player new detection DC: every 3 seconds
Player rolls d20 for stealth: every 6 seconds
NPC new detection DC: every 4 seconds
NPC rolls d20 for stealth: every 6 seconds

Note: Your Spot and Listen skills are only always active with Keen Senses or Detect Mode. Requiring caution when in Passive Detection, which will be ineffective against the quick and close Hide in Plain Sight attacks. The Detection clock cycle is also not triggered by encounters and detection may occur within the time span, whilst a trap or a hidden creature is within range. So an Assassin who has just successfully Hidden in Plain Sight may have less than three seconds to escape, before his next chance of being detected.

Search is never always active. So characters even with exceptional Search skill, should be cautious and not run when searching for traps.

Stealth Modifiers (d20)
+10 if in combat
-10 for movement (this includes cowering from bleed damage)
+5 at night
+2 Line of Sight blocked
+ Skill (Hide or Move Silent) suggest weighted average
-5 if Torch/Light carried
Relative size modifiers (Tiny: +8, Small: +4, Medium: 0, Large: -4, Huge: -8)
+/- other modifiers
Invisible (super stealth)
Silence spell (small bonus but unwise, AoE)
Detection Modifiers (d20)
+5 Detect Mode or Keen Senses
+5 for standing still
+5 if Torch/Light/Darkvision used at night 
+ Skill (Search) to reveal traps nearby
+ Skill (Spot or Listen) to reveal hidden opponents
-5 not in facing arc (erratic)
-1 for every 3 meters of distance
+/- other modifiers
Spike Growth spell (not good for your sneakers)
 
Stealth skill verses Detection skill (Detect mode, odds) rough guide, environment depending and size modifiers
-30     Player Character with Keen Sense immune to AI Sneak Attacks (or Detect Mode toggle master)
-20     Sneak Attacks unlikely
-15     Few Sneak Attacks, brief stealth
-10     Occasional Sneak Attacks, short stealth
-5       Fair Sneak Attacks, fair stealth
0        Good Sneak Attacks and stealth
+5      Reliable Sneak Attacks and Hide effectively
+10    Hide indefinitely verses Keen Sense/Active Detection
+15    Sneak effectively
+20    Sneak indefinitely verses Keen Sense/Active Detection
May require higher Listen skill to achieve the Sneak Attack immunity and note that Spot also has an advantage with detection range. https://nwn2.wikia.com/wiki/Ranges.2da Listen range is within 30ft with Spot greater than 30ft. (Detect Mode and Keen Sense are important factors in avoiding Hide in Plain Sight sneak attacks) more so than higher skill if you remain stationary and Passive. Suggest keep relocating your position, when fighting Assassin's and then select Detect Mode.
For a rough estimate to d20 verses d20 odds; 50% +/- 2.5% per DC point difference, or use the opposed skill checks table .
The +10 combat and - 10 movement is awkward compared to just +10 hiding but it does explain the quick, close combat sneak attacks. As the detected movement will be simultaneous with the attack. For estimating stealth Difficulty Check, suggest use the weighted average of Hide and Move Silently. eg 40 hide and 44 move silent is equivalent to 41 stealth, versus the expected detection skill. As the type of detection maybe either, both and often, more than one opponent. With detection only requiring one skill to be successful but stealth requiring both. (The Combined Modal Stealth DC is suggested below)  Ragimund (talk) 10:19, September 26, 2018 (UTC)

Helpful links https://www.tapatalk.com/groups/nwnecbguild/unravelling-the-world-of-nwn-traps-a-primer-t2176824.html http://nwnsotu.proboards.com/thread/356/stealth-works

Conjecture

Partial detection, the stealthed opponent is semi visible but not able to be targeted. Seems more common when hidden opponent has light source or with Listen, this maybe something like arrows being parried by the bonus shield AC. The detection has occurred by the Light penalty, therefore the light or some other factor difference in DC may have been detected and not the hidden opponent. So only aware of the targets proximity.

Did expect Listen to have better success than Spot for detection, suspect there's some bonus to Spot not applied to Listen. However Spells may favor Listen. The Listen detection range seems to also confirm with auto detect levels as always active (although only when standing still with moving target; but not in close combat or when walking), however AI sneak attackers have a distinct advantage against stationary Players without Keen Sense or not in Detect Mode (keep repositioning when fighting sneak attackers, move and then select Detect Mode).

Detect Mode (Keen Sense) has more benefit than the +5 DC (always active detection but not for Traps); especially compared to players Passively standing still (the AI is likely to select Detect Mode the instant a player enters Stealth and will have an advantage with sneak attacks against slower Passive targets). It is only in Detect Mode that Listen and Spot are always active, leaving unfortunate Passive Search characters weak in combat against the Hide in Plain Sight. So Detect Mode and Keen Senses are key counter measures against close and quick sneak attacks. Noting that the AI and some players can be quicker with Detect Mode than a player is with Hide in Plain Sight, it is then more likely to catch an opponent in Passive Search while they are fighting another opponent or in non combat actions (although this may also be considered, a Flank attack).

Number of Sneak Attacks from Stealth

Generally considered the first flurry of attacks (number of attacks + 1)/3 to the nearest. However considering that Feint, Taunt and Attacks of Opportunity are all on the clock and the number of attacks from stealth seem to be inconsistent, especially when dealing with attacks from Ethereal. It seems that attacks from stealth and ethereal are also time constrained and it just happens that a flurry of attacks may occur in the few seconds after the target is caught flat footed. (Ethereal attacks just sometimes being too slow to catch flat flooted targets). However the average number of attacks from stealth will be as per first flurry. Ragimund (talk) 11:02, April 8, 2019 (UTC)

The combined modal stealth dilemma or NWN's "hide and move silently are combined into a single modal stealth action".

Spot does work against Hide and Listen against Move Silent but this suggests two checks, however many experienced players haven't noticed their detection rate doubling with both detections skills or reason to take both. What also, if the Keen Senses/Detect mode checks are made 6 times per round as suggested.  ie 12 checks per round, each opponent. Do you know many Rogues who refuse to try and sneak up on more than a few opponents? Imagine sneaking up on a group of Elves, that'll be as effective as a marching band.

Combined Modal Stealth, can have many inputs but most likely a lot less skill checks. Individual skill checks doesn't fit the results and neither does the best or worst check. However attempting stealth is often against multiple opponents and without knowing their choice in detection. So as the actual Detection Check is unknown. Suggest use the weighted average stealth against the expected detection (assuming your skills aren't extremely unbalanced).

From a stealth perceptive, you will unlikely know your opponents detection choice but are also likely to improve your lowest stealth skill. So that's already considering average stealth versus a potential detection skill.

From the detection perceptive, you hopefully have one detection skill. It's costly doing both. Spot does has advantages with range and Feint. However, Listen may favor spells and bypass Line of Sight (although bypassing LoS isn't confirmed). Ragimund (talk) 10:09, February 5, 2020 (UTC)


Combined Modal Stealth (method that does match the results)

The character attempting stealth makes a single roll of 1d20 and adds that result to both his Hide and Move Silent skill. The opposing detection attempt also makes a single 1d20 roll and adds that result to his Spot and Listen. The character attempting stealth then compares his Hide DC against the opposing Spot DC and his Move Silent DC against the opposing Listen DC.

As most creatures attempting stealth are balanced with their hide and move silent skills, this makes detection effective with just the highest skill. However having both Spot and Listen will make detection easier of opponents with unbalanced stealth skills.

Example (ignoring the large probability curve and in game modifiers)
Hide 40 and Move Silent 40 versus 40 Spot and 40 Listen
Stealth rolls a 10 from 1d20 (result applies to both skills). Detection rolls 1d20 (result to both skills)
Spot, requires a 11 or higher
Listen, requires a 11 or higher
So if the detection roll is 11 or higher, the hidden creature is revealed. 
However if Listen was unskilled at 0
Spot, requires a 11 or higher
Listen, is unable to detect
Still with a detection roll of 11 or higher, the hidden creature is revealed.
But if Hide was less skilled at 30
Spot, requires a 1 or higher
Listen, requires a 11 or higher
Now if the detection roll is 1 or higher, the hidden creature is revealed.
ie, you need both stealth skills to be successful but only one detection to be successful

Against the example balanced stealth of 40/40, detection of either 40/40, 40/0 or 0/40 all have the same odds. ie, average stealth versus the highest detection. It's when the stealth is unbalanced that individual detection matters. ​​​​​Ragimund (talk) 09:55, February 6, 2020 (UTC)

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