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koran
10-07-2012, 20:59
Its been a long time since I have done any maths at school involving percentages I was wondering if I can get some help with some math-hammer (I would really appreciate it if the maths is also shown (or explained would be awesome!) so I can do it in the future).

I want to know which is more likely to keep the model alive:
A 4+ cover save with a reroll

of

5+ cover save with a reroll and Feel No Pain.

(Basically Im trying to decide on if I should have Karandras as my second HQ for my wraithguard army or a wraithseer).

10-07-2012, 21:03
With a 4+ with a re-roll, you have a 50% chance of passing on the initial roll, and you then have a 50% chance of passing those 50% you failed with the initial roll on the re-roll, for a total of 75% chance of saving.

With a 5+ with a re-roll, you have a 33% chance of passing on the initial roll, and you then have a 33% chance of passing those 67% you failed with the initial roll on the re-roll, for a total of 56% chance of saving. Then you have another 5+ save for a 33% chance of passing those 44% which have got through, so it brings it to 70% chance of saving.

10-07-2012, 21:08
5+ with reroll and FNP results in 19/27 chance to save, which is less than the 3/4 offered by the 4+ with reroll.

(1/3 + 1/3*2/3 + 1/3*(1-(1/3 + 1/3*2/3)))

susu.exp
10-07-2012, 21:12
And the 5+ cover + FNP is
1/3 chance of passing first cover safe
2/3 chance of having to roll the second one with a 1/3 chance of passing is 2/3*1/3=2/9
Chance to have safed up to here is 2/9+1/3=5/9, so a 4/9 chance of having to use FNP
The chance of passing FNP is again 1/3, so that adds another 4/27 and 5/9+4/27=19/27 or about 70.4%
An even simpler version of this works like this: The probability of failing 3 5+ rolls in a row is 2/3*2/3*2/3=8/27. So the chance of making at least one of them is 1-8/27=19/27.

Edit: Dang, ninjaŽd

koran
10-07-2012, 23:12
Thanks for the mathhammer guys. And even more so for the explanation, that really helps.

Now I just have to decide if its better to have the FNP for the 3+ armour saves to make them even more resilient to non-AP3 weapons or be more likely to make the Ap1 and 2 saves. Throw in that the wraithseer can have a D-cannon and its a real tough choice.

TheDoctor
10-07-2012, 23:19
With a 4+ with a re-roll, you have a 50% chance of passing on the initial roll, and you then have a 50% chance of passing those 50% you failed with the initial roll on the re-roll, for a total of 75% chance of saving.

With a 5+ with a re-roll, you have a 33% chance of passing on the initial roll, and you then have a 33% chance of passing those 67% you failed with the initial roll on the re-roll, for a total of 56% chance of saving. Then you have another 5+ save for a 33% chance of passing those 44% which have got through, so it brings it to 70% chance of saving.

Actually, I know the combo he is talking about.

For this specific combo, the Wraithseer Feel No Pain psychic power has been FAQued by Forgeworld as remaining a 4+ FnP roll. So, the Wraithseer option is slightly better in this case.

@Koran
Also, Feel No Pain works on everything that does not inflict instant death. So, in essence, Wraithguard would get their 4+FnP roll against pretty much everything.