I was reading through Time Stop spell and it says you have 1d4+1 minutes to act and I was daydreaming that you didn’t know when it ended; that’s not the RAW because in it you kinda get a feel for when it’s supposed to end, but it got me thinking to a similar issue we’ve been talking about recently. When you’re unconscious for 1d4 hours, or when you’re lost for 1d6 hours, or for how long it takes for hirelings to come to the hamlet.
Those rules are fast and work well but I was curious just for curiosity’s sake how to figure out a cumulative probability so you could make a mechanic where you rolled every hour to see if you woke up.
I headed down a couple of wrong trails early on but what I ended up with was simple.
Let’s say you have to wait/sleep/walk for X hours/days/weeks, where X is some die size like 1d6. The good old example of sleeping for 1d4 hours after getting hurt is one of these.
First, you’ve got to sleep at least one hour. What are your chances of waking up right after it? It’s ¼, or ¹⁄ₓ in the more general case. If you fail that wake up roll and sleep more, the next chance is ⅓, or ¹⁄₍ₓ₋₁₎. If you fail that, and sleep more, it’s ½ chance of waking up, or ¹⁄₍ₓ₋₂₎ in the general case. You keep going until you get to a ¹⁄₁ chance, in which case you obviously don’t roll.
To me it’s completely unintuitive that this should work, that this should give the same probs. But it does work. I simplified it backwards from a bigger equation with summed series etc, then double checked:
The same principle works for 1d6 hours, 1d12 days or whatever. It looks bloody obvious to me now that I’ve spelled it out like this and I’m probably gonna get a lot of “Well, duh…” but my first intuitions of how a mechanic like this would look were quite far from working out and it was third times the charm as I finally worked it out and then I reordered it and got it in this order and was like “Ok, yeah, then duh… now that I see this…” It was sort of a Monte Hall problem for me until everything clicked into place.
So in practice for sleeping 1d4 hours:
For 1d4+1 minute, just go through an extra minute before you start.
For being lost for 1d6 hours:
(It’s pretty much 1d6, 1d5, 1d4, 1d3, 1d2 and then a sure thing.)
For a d20 equivalent, it’s 11+ (2 hours), 14+ (3 hours), 16+ (4 hours), 17+ (5 hours), 18+ (6–8 hours), 19+ (9–13 hours), 20+ (14 or more hours).