I was trying to calculate some probabilities, but my Disrete Math is too poor.

When making a Psychic Power roll, what are the odds that at least one die could roll 9? How does this change from 1-6 dice?

Also, some people have mentioned in other threads that you can use fate points to mitigate or help avoid Psychic Phonomena. How is this done? I cannot find the specific rules.

Thank you for the help.

If one dice has a 10% chance of Psychic Phenomena, then rolling 6 dice gives you a 60% chance of at least 1 of them coming up a "9".

Essentially think of it as rolling the same dice. If you roll it once, then you have a 10% chance of getting a 9. If you roll it twice then you are exposed to that risk twice, so its 10% + 10% equals 20%. If you rolled the dice 10 times, then assuming an average, random spread of results (1, 2,3, 4, 5, etc) then you'd be at 100% risk of getting a 9 at least once.

SJE

Hate to contrdict, but that's not how it works. If there was 100% chance of getting a 9 with 10 dice, that means you would always get one, always. And we all know that's not the case.

Pulling out the several semesters of statistics I had to take back in college: Although counter-intuitive, the way it actually works is that you subtract the chances of the event in question not happening at all.

So your input into this calculation is the idea that on one die, you have a 10% chance of a nine, and a 90% chance of no nine.
So on any given roll, the probability of a 9 is 1 minus( the probability of rolling no nine on all the dice rolled.)

Chance of getting a nine with one die: 1-90% = 10%

Chance on 2 dice: 1- (.9 * .9) = 1-(.81) = 19%

Chance on 3 dice 1- (.9^3) = 1-.729 = ~ 27 %

and so on...

You are quite right- calculate the probability of it not happening is the way to go-

1 dice- 10%,

2 dice 19%

3 dice- 27%

4 dice - 34%

5 dice- 41%

6 dice - 47%

Me am dumb. Though I still hold that if you roll 10 dice, most often a 9 is going to creep in there.

SJE said:

*lol* On that, I definitely agree.
Just for those following along at home, 1-.9 ^10 = 1- .359 = ~ 64%

Thank you both.

And what do people mean whan they talk about using Fate Points to avoid power related mishaps?

House Rule to avoid the possible game ruining absurdity that can be spawned by too many random charts. (reminds me of the D&D Days of 'weird occurence spawned the gazillion tables...)

I think its a house rule where GM's let the psykers re-roll a 9 if they spend a fate point. Not sure if its all the dice or just the one that rolled 9.

Should make for a less frightening psychic use. Not sure I agree with it, but I'd have to test it in play.

SJE

Okay. I don't think I'll be able to introduce that rule, seeing as a player in our last game (who is no longer playing with us) did that abusively for months despite player and GM protest. That and other rules bending silliness soured some of us on Psykers and made his character vastly overpowered. Still, I hope I can play one enjoyably in our new game.

I thank everyone for their help.

(As a house rule) I allow psykers in my games to spend a fate point to re-roll a result on the Perils Chart. Not the occurrence of a 9, or the roll on the phenomena table that caused perils, just the outcome on the perils chart.

Nothing on the perils chart is good, so I don't find this too unbalancing. But it is a decent way to minimize truly game-derailing results.

aethel said:

I can see that as a valid point of view, and likely helpful.

However in my campaign a botched perils roll (daemonic possession) actually resulted in the generation of some of the most dynamic plotlines in the game so far.

The characters where in the main Adeptus Arbites Precinct Fortress on Coseflame. Our psyker got possessed. The randomly rolled-up unbound daemon host came out crazy crazy powerful (and thus was designated Hatherack, Herald of Tzeentch), and went on a carefully planned rampage (only killed every 9th arbitrator, "hail Tzeentch!"), then cornered one of the other acolytes (our arbitrator) . The daemon then made a pact wherein the cornered acolyte would give up his soul, and serve the daemon, in exchange for the power to survive and to smite his enemies (specifically to avenge himself on his father's killers, but when the player realized just what he was giving up [his soul and freedom] he decided to try to bargain up for smiting all foes in general). Then the daemon (which has an Int score in the 80's or 90's IIRC) rigged the power reactor in the fortress into a fission bomb and blew up the single militarily strongest Imperial facility on the planet (thus changing the socio-political power structure of the whole planet. "hail, Tzeentch!"). Needless to say the acolytes who escaped the resulting blast (including the one who made the secret pact) gave up on the rest of what they were doing on Coseflame and got off world ASAP. The result of this being that when they see their Inquisitor again and are debriefed (right after the adventure I'm currently running) the next mission will be to head back to Coseflame and kill or capture the Daemonhost.

Of course even if we had used your house rule, it wouldn't have changed the outcome of this craziness because the psyker in question had just burned (not speant, burned) his final fate point right before rolling up getting posessed.

Sure the lunacy from perils derailed this one adventure, but the story of the corruption of Nihilus Eisen (our "arbitrator") [at 30+ CP's and climbing, just made his first rolls to resist mutation during yesterday's session] has become one of the most imporant ongoing subplots in the campaign. Also it is providing the set up for a great mission to redeem the acolytes' inadvertant dooming of a world.

So crazy random perils not necessarily bad if the GM and players are prepared to roll with it. Can make for good dramtic storytelling.

DocIII said:

Also true. I actually have a caveat in the house rule that the GM may override your ability to re-roll. (I was just trying to be breif in my prior post.) There are some results that at specific times could be disruptive without being entertaining, and it is really those I am trying to avoid. I mean, I suppose the alternative is just to roll the perils result in secret and fudge it when necessary, but not doing so is a personal preference thing on my end.

That is the Goldern Rule of Random Charts as far as I am concerned. I never let me players see charts that they're rolling on and some are multipart affairs, they never know when though so if something that doesn't seem apropriate or seems likel it might break the game or ruin the scen I just say somthing like, 'OK make another roll, and one more...' ect so as to keep some randomness in games but not to ruin it.

And while I agree in part with DocIII as I've had a simmilar experience as him only in my WFRP game, it can really be nice to roll with a series of terrible rolls or bizarre events that can just spring up, however I've found in my years of gamming that the 'game killers' seem to far ooutweigh the 'game makers' I recall hearing tales of entire cells wiped up in first session of play because of a bad perils on silly narative powers like Knack or Lucky, that's just bloody shite there. GMs should use 'Common Sense' on these things, or just make their own charts (which is the route I've gone.)

Making judgment calls and functioning as both referee and review board (to decided what rules are used and when and how interpreted) fairly is at the core of what it means to be GM in a game, rather than just freeform storytelling (nothing against freeform storytelling, just distinguishing between a game played using story and just telling stories, both of which are valuable, just a bit different)