On 24 May 2014 10:32, Postmark <postmark.design@btinternet.com> wrote:

On 24 May 2014, at 08:40, Timothy Collinson <timothy.collinson@port.ac.uk> wrote:

Also the price ranges won't be big enough - I looked up the price range for a Ford Ka vs a Ford Mondeo and it is nearly 4x between the cheapest Ka and the most expensive Mondeo.

So on the plus side making them each 5% isn't so far off. On the negative side it doesn't really work. Hmmm, back to the drawing board. Or is this why Marc didn't include such a factor in the Core book?!

I was actually thinking of multiplying the percentages so +1 +1 +1 -1 would be 1.05 x 1.05 x 1.05 x (1 / 1.05) =1.1 and +5 +5 +5 +5 would be (1.05)^5 + (1.05)^5 + (1.05)^5 + (1.05)^5 = 2.67.

-5 -5 -5 -5 would be (1 / 1.05)^5 + (1 / 1.05)^5 + (1 / 1.05)^5 + (1 / 1.05)^5 = 0.367

Ah, ok. With the exception that the fourth placed digit (Burden) is reversed for adding or subtracting (as a negative burden is a benefit), I think I'm following that. Although to be fair it looks very similar to my Plan B where each QREBS value point was a 5% change. (If I've got it right 1.05^5 is around 27% whereas my 'simple' count makes five points = 25%). (My apologies if I'm failing to understand something basically mathematically that makes these different.)

Those don't seem enough to me, so try 10%, which gives 1.21, 6.77 and 0.148 as as the multipliers.

Ok, although I'm still not convinced 0.148 isn't somewhat too cheap for something that's new. Even if its rubbish!

[(1.05)^5 meaning 1.05 x 1.05 x 1.05 x 1.05 x 1.05 in case the notation is unfamiliar.]

Yes, happy with the ^ notation. (But thanks for checking)

Possibly the minus side could be a different percentage to the plus but that would mean that the baseline couldn't be chosen arbitrarily

Well, again to keep it simple, I was assuming I'd take the baseline price as the rule book price. I'm not sure I want to go through equipment guides etc giving everything a new baseline price just to work with this system. I want to keep it as easy, memorable and as usable across the various rule sets as possible. Which may be a pipe dream I know!

- if you have cheap, mid-range and top-end cars then you want to generate the same prices by setting the cheap price and having the values as 0, +1, +2 or by setting the mid-range and having -1, 0, +1 and that means plus and minus need the same ratio

I'm not sure I understand this last section. Unless you mean that you'd have three different baseline prices depending on whether we're looking at cheap, mid-range or top-end. I was thinking that the cheap, mid-range and top-end prices would be generated by the system, not that I'd have three different baselines for a product and start from there with the QREBS alterations.

But many thanks for helping me think through the issues. It looks as if I'm just going to have to pick a method and go with it as I think either way will give me roughly what I want. I'm sure other referees will adjust as they see fit in any case!