People don’t realize, but AD&D has always had a Perception system and Perception checks, if you check out page’s 59 and 60 of the Dungeon Master’s Guide it has rules for both “Detection of Invisibility” and “Listening at Doors” that apply to all classes – outside of the Surprise rules or the Thief’s ability to “Hear Noise”.
This also ignores the whole set of special racially-based detection abilities of Elves, Half-Elves, Dwarves, etc. and the rules for detecting Poison in the Player’s Handbook…
First off, it says that Humans, Dwarves, and Half-Elves has a base 10% chance to Hear Noise, Elves and Half-Goblins have base 15%, and Gnomes have a base 20% (which matches the bonuses in the Player’s Handbook for racial bonuses to Hear Noise). Furthermore, at character creation roll a d20, on a 1 you have a +5% and on a 2 you have +10% to this base chance due to “Keen Hearing”. There is of course, no statement as to how this applies to Thieves…
Furthermore, in the Detection of Invisibility table it is a function of Level or Hit Dice as indexed with Intelligence on a matrix – starting at 17+ Intelligence and a 7th level character having a 5% chance to Detect the Invisible. At 15th level this character will have a 95% chance, and the progression is rather clunky and uneven across the matrix. But according to this, a character of average Intelligence will have about a 5% chance to Detect the Invisible roughly around name level and will have about a 50% chance at 15th level and higher.
Wouldn’t it be nice if we could just integrate these two things into one damn table with a more even progression?
Perhaps even something that might grant some of those of those Roguish types something even a bit more to make them a tad bit more special that just a fighter?
Perception / Hear Noise: Base 10% (Note some races have different bases)
- Roll 1d20 at Character Generation, on a 1 you have Keen Senses and gain a +5% to that Base, on a 2 you have Very Keen Senses and have a +10% to that Base. (Note, this also gets used when checking Surprise)
- Rogues and Warrior Monks get +5% for every odd level.
- Entertainers and Psychics get a flat +10% to the Base.
If attempting to Detect the Invisible, characters add their Level (or creatures their Hit Dice) to their Intelligence score and multiple the result by two, they then add this to their normal Perception / Hear Noise percentage. Penalize it by -60% (-30% for Name level characters or higher), and this is the chance to Detect the Invisible.
When there is the chance to Notice Poison, easy checks (poison on a blade) tend to use the normal Perception base while determining if food or drink has been poisoned generally uses same percentage as Detecting the Invisible. This is a non-cumulative roll, and is instead merely checked against the base each relevant interval of time (usually per round of exposure).
There, now that is a simple and unified system rooted in the Dungeon Master’s Guide ideas and rules. You can use it to roll on all sorts of Perception checks if you want, but between this and the Surprise rules, you have pretty much everything you might need to figure out what people notice, and how surprised they are if they don’t. All of these percentages can be adapted to use for other related situations, and all of them can be modified up or down as the DM sees fit depending on the circumstances. I don’t tend to modify them down, but am more like to have a character roll and see how well or how badly they make it in order to dole out less or more information – but that’s also my DMing style.