The clue probability rule

(I'll return to posting gameable content shortly.)

Most everyone on the internet is familiar with Alexandrian's three clue rule. The stipulation of that rule is that players are likely to miss some of your prepped content, which also includes any given clue. So to give the players a decent chance of progressing through a investigation-adventure, you need more than one clue leading to the conclusion, or to any breakthroughs that eventually lead to the conclusion. True to its name, the rule states that the perfect number of clues for each breakthrough is three.



I like this rule, for two reasons.

First, it means that each breakthrough is overdetermined: there is more than one path leading you there. This, to me, was a central thing in Masks of Nyarlathotep. You had a hunch from the beginning that you were going to Africa, but it felt very distant and hard to justify - what were you going to do there? With each new clue, you became more and more confident that you were actually going to Africa, up until the point where it seemed like the only reasonable thing to do. So basically, each new clue reinforces those you already have. Maybe you were already sure that Mr Vardis was a killer, but confronting him at this point seemed to carry so high costs. And then you learn that he is about to strike again, and suddenly acting becomes more urgent.

Second, and conversely, it also gives the adventure a branching structure. As there are always too many leads to follow up, the players will have to prioritize and consider what-if scenarios. Once the killer is caught they have perhaps seen 75% of your prep, so they still might not know what was the deal with Mr Vardis' prize-winning kennel. This makes the world seem bigger and the mystery more engaging.

In sum, the three-clue rule recasts a linear adventure to a dungeon-structure, where clues lead to new locations and breakthroughs in the investigation much like doors and corridors lead to new rooms.

However, this also shows some problems with the three-clue rule.

If we take the dungeon analogy seriously, we should accept that it isn't just a version of a linear adventure. Most dungeons don't have a straight-line progression to the end. And most dungeons are ok with you not reaching the end. If we were to apply these aspects of dungeon design, there would not always be three clues leading to the next revelation. Often there might just be one, and sometimes it would be hidden. And if the PCs don't find it they'll have to backtrack or even give up. And other times there would be clues that lead more than one step ahead in the investigation. So instead of 3 clues leading to A where 3 new clues lead to B etc, you'd have something more akin to a web.

However, even if we switch up our three clue investigation in this way - making sure that there are not just clues leading from A to B but also clues from B to A and from A to B via X and Y - we run into the problem of direction. Once you've reached C, it makes little sense to return to A because C is closer to the final conclusion than A. This is the same as in a dungeon, where you progress from the point of entry towards the central treasure through deeper and deeper levels.

If you want an even greater level of freedom in your investigation, your clues must be meaningful even in the presence of other clues. This is the rationale behind the guess-who structure and the triangulation (or venn-structure, as Kyana pointed out), described earlier. Here, all clues carry equal weight so as long as you're still uncertain about the solution, you gain from seeking and finding new clues.

For this type of investigation, I'd like to propose another rule for clues: the clue probability rule.

For each conclusion that you want the PCs to make, include a clue that they have a probability of finding whenever they make an effort to find it.

Say that you have a murder weapon: a knife with a easily recognizable look that would lead you to the smith who made it whom in turn could provide a list of clients. The knife must be somewhere. So we decide that it is in the pond, close to the scene of murder. Now if the PCs would look in the pond, they would definitely find it. So the clue probability is 100%. But perhaps they don't, because how would they guess that it is there? Instead, they might investigate the wounds. They can determine it's a knife without problem, but could they also conclude that this type of knife is made by Brambly the smith? I dunno. 1 in 6. Otherwise they just learn that the knife must have curious proportions. So maybe they ask a smith, or an assassin for help. 1 in 3. Or ask witnesses. 50% chance. The scrap-finder kids or pawn shops? 10% per day has passed since the murder. And so on.

The point here is that instead of designing several clues leading to the same conclusion (or in addition to), you design a clue that there's always a chance of finding whenever the characters make an effort to find it.

So now we have a hierarchy of blueprints for investigation adventures

The linear structure. A breadcrumb trail where A leads to B leads to C. Fail-safe if clues are not hidden behind roll to continue. Very little agency, built in story arc.
The three clue structure. A breadcrumb trail where multiple clues lead from A to B and multiple clues from B to C. Almost fail safe, given the plurality of clues to each conclusion. Some agency, allows for story arc.
The dungeon structure. A web of breadcrumb trails leading between A, B and C through multiple clues. Failure is unlikely. Good agency, story is largely emergent.
The open structure. A limited set of clues of equal (or similar) importance, which can be found through many different approaches. Failure is possible. High agency, no story arc.

Map + clues

Here's my campaign map, or the SE quadrant of it.  New things are added as I prep them.

My campaign consists of three layers of adventure, superimposed.

At the bottom layer, there is a square crawl. Given the vast number of possible location, any content existing on this level is very likely to be missed. Because of this, the square crawl basically relies on tables for content. This means that I can have a dozen towers on the map and only have content for six of them, or hundreds of wilderness-squares and only content for twenty. So far, my players have engaged with the square crawl exactly once so this seemingly limited content is actually plenty. If I were to run out, I'd write more. Until then, it's just a waste of prep time - especially since d4 Caltrops got everyone's back.

At the second layer, there is a point crawl of locations connected by roads or trails. This is what my players mostly engage with so this is where I focus most of my prep. At the points of the crawl are adventure locations, set piece encounters, small scenarios or key NPCs. Here I also slot in adventures made by others when I find some that fit my aesthetics and tone well enough. So we've played Curse of the Shrine Goddess in this way, and I've placed a couple more.

The final layer consists of an investigation-mystery that provides the default point of engagement. This is basically a poor-man's CoC, with handouts containing clues for the central investigation into the relics of St Severend/Severinus as well as other mysteries more related to the history of the campaign setting (Why did Severend's holy mission fail? What happened to the Witch King? These kinds of things).

The third layer is the reason for my interest in different clue structures. Previously, I've outlined the elimination or guess-who structure where each clue cuts the remaining possibilities in half. This allows you to eliminate options until you eventually only have one left, which is the correct answer. The method have survived the first contact with playtesting: in Sourn, my players DID figure out the who the murderer was in about two hours without any pointers from me, DID NOT perceive the underlying structure, and were consequently super impressed with their detective skills and their luck. A great success (and a secret they must never know)!

So with the guess-who structure seemingly doing its job, it's time to introduce a second structure: the triangulation. The idea of triangulation is this: if you don't know where you are but can observe some landmarks whose position you are familiar with, you can use their bearings to determine your own position. So if your first clue is "east of X" and your second clue is "south of Y", you basically have the row-column coordinate of the thing you are looking for. You could also use it for social relations: "relative to Mr X" and "co-worker of Ms Y".

Here's a version, using distances instead of directions, which creates two circles with the target at their intersection. (Note that unless you're super skilled at measuring you'll probably end up with circles that intersect twice, so you'll need three distances or some constraining factor). Travel speed is 18 mi per day. 

The Saint's Hand 
No sooner had Severinus died, than many accounts began to come of miracles because those who touched his severed hand regained sight if sightless or speech if speechless, and those who had terrible coughs could again draw breath like young people. After five days at a stake in Mersault, the hand was taken down in secrecy and smuggled to a tomb seven days’ travel by road from there, and Ygdrain was powerless to find it. 

 “A swollen tongue” – Excerpt from traveler’s log 
At one time my tongue became uncomfortably swelled up, so that when I wished to speak it usually made me stutter, which was somewhat unseemly. I went to the tomb of the Saint’s Hand, eighteen miles from Croix-an-Tour as the bird flies, and drew my awkward tongue along the wooden lattice. The swelling went down at once and I became well. It was a serious swelling and filled the cavity where the palate is. Then three days later my lip began to have a painful beating in it. I went again to the tomb to get help and when I had touched my lip to the hanging curtain the pulsation stopped at once.