Talk:Numeric error propagation

Task updates

Unfortunately I probably won't be able to track comments on this draft task for a couple of days as I will be making the most of a weekend with my beautiful partner without our kids. For what it's worth, I had hoped to make this task rather like Quaternion type. (Maybe I should change the name to "Error propagation type")? --Paddy3118 02:59, 30 July 2011 (UTC)

What about correlations?

Even if error of a and b are fully independent, stuff like (a+b) and (a-b) would be (anti)correlated. What to do with those? Ignore? --Ledrug 03:00, 30 July 2011 (UTC)

I purposefully skipped the stats and just extracted some equations; hopefully enough to make an interesting task. I think it might be best to leave such extensions to the individual language implementors and have them make notes in their language entry?

I don't want to further expand the task description with talk of correlations unless their omission really offends those in the know. (University was a few decades ago for me and I haven't made much use of the subject since). --Paddy3118 03:13, 30 July 2011 (UTC)

As long as the task doesn't require operations between correlated errors it doesn't make a difference. Otherwise each uncertainty would have to keep track of the error sources, which will become complicated real fast. --Ledrug 03:22, 30 July 2011 (UTC)

Not sure how to do this

Conceptually speaking: the new type is a number plus a function of a random variable.

But consider, for example: a*a. Here, both factors depend on the same random variable. We could express this particular example as a^2, because the same random variable is being used on both sides of the equation, but in the general case?

I am not seeing how to make this consistent, except by pushing this burden of analysis onto the programmer. But that approach seems at odds with implementing this as a type. --Rdm 15:55, 31 July 2011 (UTC)

They should be treated as independent for the purposes of this task, but see the Python solution where I have added a check for an object being multiplied by itself, which means having the same id() in Python (implemented as a check on the memory location occupied by an object in C-Python). --Paddy3118 18:19, 31 July 2011 (UTC)

Don't do that. This is the same question as the one I asked above about correlations. For a variable, you either track all its error sources for correctness, or track none for simplicity. Checking just one source is good for neither. Consider:<lang>a.value = 10

a.error = 1.0 b := a * 2 c := a - b</lang>How do you deal with the error at assignment of c and d? Checking memory location isn't enough. If a variable has full error source information, a and b would have the same source of error but with different magnitude, so c.error == a.error, but a and b won't have the same mem location so you probably would think c.error = a.error * sqrt(5). Checking identifier equality is not much more correct than tracking nothing, but more confusing and less consistent. --Ledrug 20:20, 31 July 2011 (UTC)