Do I have the probabilities right?

[Geoffrey]

Please be patient with me, as I've never played 4E. I've only studied the PHB, DMG, and MM.

Let's say that a deathjump spider (MM, page 246) bit your character. He would take 2d6 + 3 damage. Each round thereafter your character would be in danger taking 5 more points of poison damage. To avoid this 5 points of poison damage, you character would have to make a saving throw by rolling a 10 or higher on a d20. Each round you failed to roll at least a 10, you'd take 5 points of poison damage. Am I understanding all that correctly?

I've (inexpertly) worked-out percentage chances of failing successive saving throws. Are these percentages essentially accurate?

55% of the time, your character would fail 0 of the saving throws.
29% of the time, your character would fail 1 of the saving throws.
9% of the time, your character would fail 2 of the saving throws.
4% of the time, your character would fail 3 of the saving throws.
2% of the time, your character would fail 4 of the saving throws.
1% of the time, your character would fail 5 of the saving throws.

 

[Ferghis]

No. They are, in part, wrong.

The probability of making the first save is 55%. The probability of failing the first one and making the second one is (0.45*0.55=) 24.75%. Further fails just increases the number of 0.45s in that formula. So the odds of failing two saves and making the third one would be (0.45*0.45*0.55=) 11.375%. The odds of failing three saves and making the fourth are 0.45*0.45*0.45*0.55=) 5.011875%. And so on. If you turn this into a table, and never round, you should never be able to add all the percentages to 100%, unless your table is infinite. The total will keep growing closer and closer to 100%. That's because there is always a chance that you could keep failing the save. Forever.

 

[Journeymanmage]

...
To avoid this 5 points of poison damage, you character would have to make a saving throw by rolling a 10 or higher on a d20. Each round you failed to roll at least a 10, you'd take 5 points of poison damage. Am I understanding all that correctly?

I've (inexpertly) worked-out percentage chances of failing successive saving throws. Are these percentages essentially accurate?

55% of the time, your character would fail 0 of the saving throws.
29% of the time, your character would fail 1 of the saving throws.
...

55% of the time, the character would fail 0 saves
45% of the time, the character would fail 1 save
20% of the time, the character would fail 2 saves
...

 

[Aulirophile]

(Save Ends) means the power is over, you never take damage from the ability again, unless it is re-applied of course (by another successful attack from the Deathjump). So the question is phrased improperly, the math wouldn't work that way.

It'd be 0.45^number of failed saves*.55 for the probability of each save. I say each save rather then each round because there are a large number of ways to generate saves outside of the automatic end of turn ones. B