Possible sampling error

[OAGr]

From a intercom request:

' There’s currently a bad bug though in how chains of calculations are dealt with. As it stands, in a chain of calculations X -> Y -> Z, it appears that random numbers are sampled for X to calculate distributions in Y, but then to calculate the distribution of Z the values of Y are resampled. The effect is that correlations are ignored entirely. What should happen is that random numbers are sampled from X and used to calculate the whole chain of values (Y and Z). Here is a quick proof of the error: http://getguesstimate.com/models/2218. For two data cells I’ve labelled A and B (e.g. X = {A,B}), there are separate cells for C=A/(A+B) and D=B/(A+B) (so Y = {C,D}), and finally a cell that is the sum of the two fractions E=C+D. This should be identically 1, because A/(A+B) + B/(A+B) = 1. Instead, an error range is found for E because the values for C and D are resampled ignoring their anti-correlation. Doing the entire computation in one step, shown in the cell labelled F, works correctly because the values sampled for X={A,B} are used directly in the computation of F=A/(A+B) + C/(C+D). '

[dfrankow]

Closed because .. it was resolved?

[mmcdermott]

Yep! This problem should not happen anymore. In particular, if you look at your test model in the issue, neither E nor F has any uncertainty.

[dfrankow]

Thanks!