Generating psychic powers -when to choose Primaris option

[Denzark]

Hello all

I have been re-reading psychic powers in particular the generation of powers, due to the imminent inclusion of Typhus in one of my armies. Now, most normally, I have seen my opponents roll say, for warp level 2, 2 dice, and make sure the choices rolled are both different, and then choose whether or not to opt out for the primaris of one or the other. I have come to the conclusion, that this is actually wrong and disadvantages the opponent.

Why?

The BRB refers to choosing the primaris power IMMEDIATELY after the dice is rolled. I can't be arsed to mathhammer the difference for here, but say your choice of powers, has one negative choice, and you roll it first dice. If you DID IMMEDIATELY choose the primaris power as a substitute, you then have equal odds of the second roll, being your negative choice - but you don't have primaris left to swap it out for. If you simply roll a dice per warp level, re-roll duplicates, andthen choose, you have far more flexibility and I believe are less likely to roll a negative choice.

It may be that people are playing it correctly anyway, but I have only just noticed and it is more normal that I am on the receiving end of psychic shenanigans - so this could be a little support against all the re-rolls to hit etc that seem to prevail in the realm of psychic mind bullets.

Comments appreciated,

[Magpie]

Yep the correct way is that you have to make the choice of rejecting the rolled power and taking the Primaris power before you roll the next one. Certainly rolling all your powers and then deciding which one to reject is wrong.

If you roll them both together you don't have that conundrum of "will the next one I roll be better than the one I just rolled". On the other hand tho' if you roll a power you don't want on the first roll, if you reject it and take the primaris power then you have a 1 in 6 chance of getting the same power again but if you don't reject it you get a 1 in 5 chance of getting the power you want, because the power you already have can't be taken again.

[Denzark]

I think in some cases the negative is more likely - I mentioned Typhus, he can only roll on the Nurgle table (quite rightly) so I need to get it right.

[mathhammer]

It also effects if your going to roll on a second table. If i'm replacing 4 powers and if i could roll two powers at once it would make it better for me.

Thus the rule book says one at a time which makes the choice a lot more difficult.

math wise rolling multiple powers at once is a big boon. The primary case i see is:

Correct way:
Die roll -> 5
Swap for Primaris
Die roll -> 5
Result Primaris and power 5

Wrong way
Die Roll 5,5
Reroll duplicate X
Swap 5 for Primaris
Result Primaris and power X

So it lowers the choices for the second die by 16% ergo raising the odds of you getting the power you want on the second dice by (1/6 + (1/6 /5)). This is caused by the Primaris power swapping. If there wasn't swapping then the odds on the second die would be the same if roll together or one by one.

[Magpie]

This becomes doubly bad when you see the Nurgle table only has 3 psychic powers + the primaris, the odds become 1/3 of getting the power you want.