New trait math

[nick_coffin]

When creating a new trait by taking an existing trait and applying modifiers, when do you round the fractions?

For example, I want to build a trait based on Burning Attack 1d-1, with +855% worth of modifiers. Burning Attack 1d-1 is worth 3.5 points before rounding. Is the cost of the trait 3.5 + (3.5 x 8.55)? Or is it 4 + (4 x 8.55)?

[Varyon]

Round at the end - it’s 3.5*9.55 in your case, not 4*9.55. Personally, I feel even when building a new trait by combining (possibly-modified) existing traits, it’s appropriate to round at the end, but RAW would round for each component of the metatrait. I’m pretty certain the rounding rules are more for simplicity than game balance anyway (fractional points make things more complicated).

[namada]

As per B101, top paragraph in the third column, Base Cost x Net Modifier, then round.

[nick_coffin]

Originally Posted by namada

As per B101, top paragraph in the third column, Base Cost x Net Modifier, then round.

Yes, I saw that. That assumes that the base cost of the trait is the fractional cost. I could also assume that the base cost is the full cost of the trait if unmodified. In this case, using my logic, the base cost is 4, not 3.5, since you can't have a fractional cost for a trait.

I'd like to know what Kromm thinks. Or if someone can point out an example that makes the math clear.

[Celjabba]

Kromm answer :

http://forums.sjgames.com/showpost.p...53&postcount=8

You might be thinking of enhancements and modifiers . . . which are always applied to final cost after buying N levels, and never, ever, for any reason applied to per-level costs.
Should an advantage actually yield a fraction before modifiers, you're meant to round the fraction up and then apply the modifiers and round that result up.

See also

http://forums.sjgames.com/showpost.p...13&postcount=7

Burning Attack 3d+1 consists of 3 + 0.3 = 3.3 dice, which costs 3.3 × 5 = 16.5 points, which rounds up to 17 points.

Affects Insubstantial is a +20% enhancement. Applied to the Burning Attack above, it multiplies cost by 1 + 0.2 = 1.2. Cost becomes 1.2 × 17 = 20.4 points, which rounds up to 21 points.

If Burning Attack 3d+1 (Affects Insubstantial, +20%) [21] is an alternative ability of some sort, its final cost is 21/5 = 4.2 points, which rounds up to 5 points.

[TGLS]

Hm. That contradicts Psionic Powers then; TK bullet costs 5 per level despite being built as Peircing Attack 1d-1 with a 40% net modifier. Rounding only at the end gives 5 (5*0.7*1.4=4.9) while rounding after both steps gives 6 (5*0.7=3.5, 4*1.4=5.6). Though Psionic Powers has a little fudge to begin with.

[Celjabba]

Answer is on page 53 of Psionic Powers :

In GURPS, the value of an Innate Attack that causes partial dice of damage (see p. B62 for details) is calculated as follows.
1. Figure out how many effective levels of the Innate Attack are
being bought (e.g., a 3d-2 attack equates to buying 2.4 levels).
2. Multiply the per-level cost of the Innate Attack by the effective
number of levels.
3. Round the cost up to the nearest point.
4. Apply the net value of all modifiers.
5. Round the cost up (again) to the nearest point.

The write-ups in this book intentionally skip step 3.

P.K. fudged the calculation because in Psionic power you don't look under the hood and only look at the price per level. Which that way can remain constant without overpricing at higher levels.

[nick_coffin]

Originally Posted by Celjabba

Answer is on page 53 of Psionic Powers :

P.K. fudged the calculation because in Psionic power you don't look under the hood and only look at the price per level. Which that way can remain constant without overpricing at higher levels.

Psionic Powers is one of the few Powers books I don't have. :-(