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new values for tolType flag with Math Objects

new values for tolType flag with Math Objects

by Alex Jordan -
Number of replies: 3
I'd like to make a third and fourth option for the tolType flag in Math Objects. It appears to me that I would have to make alterations to each of pg/lib/Value/Real.pm, pg/lib/Value/Formula.pm, and pg/lib/Value/AnswerChecker.pm. Is this right? And is this the extent of where such additions would need to be made, or are there more places that would need attention?

That's my question. For the curious, here are the options that I am thinking of.

The third value for tolType would be sigfig, where comparison would be based on having the first tolerance significant digits in common. tolType=>relative almost does this, but it's not the same. This would agree more with how human graders tend to grade precision.

The fourth value would be percentage. I haven't fully thought this one through yet. To mimic how human graders grade percentages, its almost like you want relative or sigfig for tolType with answer close to 0, but for answers close to 1, you want the same kind of checking done with the complementary percentage. For example, if the answer is 0.9995, then relative tolerance alone with 0.001 for the tolerance would make an answer like 1.0004 correct. But if we were grading using relative tolerance on the complement 0.0005, this would not be the case.
In reply to Alex Jordan

Re: new values for tolType flag with Math Objects

by Davide Cervone -
Those three files do look like the only ones (in MathObjects) that use the tolType value directly, so they seem to be the right places to change. Since this is such a fundamental part of WeBWorK, however, I wouldn't be surprised if something didn't pop up elsewhere.

I think sigfig is a nice idea. Can you say how you plan to implement it? There are undoubtedly numeric subtleties to be considered. I think it would still have to take the zeroLevel into account, since values that should be zero but have round-off errors tend to have no useable significant digits.

I'm not sure I understand your percentage value. Can you explain more about what you have in mind, and where it would be used?
In reply to Davide Cervone

Re: new values for tolType flag with Math Objects

by Alex Jordan -
Before machine arithmetic considerations, suppose with sigfig, we wanted to compare x to y. Looking at y, floor(log10(y)) tells us where its first significant digit is. For example,

floor(log10(0.234)) = -1
floor(log10(2.34)) = 0
floor(log10(234)) = 2
floor(log10(100)) = 2

So then if tolerance is say 3 (for requiring 3 significant figures to be correct) then compare x and y using the difference between x and y, with an absolute tolerance (different from the sig fig tolerance value) of 5*10**(floor(log10(y)) - 3)

for example, in comparing x to 0.0234 with 3 sigfigs, x would be equivalent to 0.0234 if
abs(x - 0.0234) < 5*10**(floor(log10(0.0234)) - 3)
abs(x - 0.0234) < 5*10**(-2 - 3)
abs(x - 0.0234) < 5*10**(-5)
abs(x - 0.0234) < 0.00005

This might need some tweaking if we use 3 sigfigs, but the y in the example is something like 0.02344444... I haven't thought it through completely. But basically you are using an absolute tolerance comparison, where the absolute tolerance level changes depending on the order of magnitude of one of the numbers to compare.


With percentages, consider a context where only answers in [0, 1] are appropriate in the first place. Like computing a probability. (In fact, 'probability' might be a better name for the tolType.)

With tolType => absolute,
With tolerance => 0.01, a common correct answer in stats of 0.997 would let 1.000 or 0.999 get credit. While they are numerically close, they are galaxies away in a deeper sense. For answers near 100% or 0%, you tend to want high precision.
One could alleviate this by using tolerance => 0.0005, but then what if the problem allows the same answer variable to be 0.5123? We simultaneously don't want to force students to use all that precision when the answers are not close to 100% or 0%. In this case I think that many would be happy with answers of 51%.
Worst of all, no matter what you may use for tolerance, including 0.0005, what if the answer ends up being 0.999992? Is it OK to answer 100%? With 0.00003, is it OK to answer with 0%? Both feel wrong, since in this context 0% and 100% are so different from anything else.

With tolType => relative,
With say tolerance => 0.001,
Then these issues go away at the end of the spectrum close to 0%. For an answer close to 0%, you won't get away with 0% until all these numbers are down so low that we are at the zeroLevel.
But the issues are still there at the end of the spectrum close to 100%. Now a correct answer of 0.9992 would allow 100% to be counted correct.

So in the end, when comparing the student's x to the answer y, what feels right is to:
  • use a relative precision (or sigfig precision) comparison of x and y when y is less than 50%,
  • use a relative precision (or sigfig precision) comparison of (1-x) and (1-y) when y is greater than 50%
  • or maybe for simplicity, declare the values equivalent if they pass two comparison checks: one of x with y and one of (1-x) with (1-y), regardless of position relative to 50%.
In reply to Alex Jordan

Re: new values for tolType flag with Math Objects

by Davide Cervone -
I figured your approach for sigfig would be the one you outlined (though you would need to take negative values and zero into account when taking the log, of course).

This is definitely the mathematicians approach, but I think there is an easier way, which lets perl do most of the work. If $n is the number of required digits, then compare $a and $b via

    my $format = "%."+($n-1)+"E";
    sprintf($format,$a) eq sprintf($format,$b);
This asks perl to format the numbers using exponential form with n-1 decimal digits (and one leading digit, so n total digits). This avoids the numeric issues, the issues of negatives, and so forth.

In terms of the percentages, I think this is a special case that should be handled as a separate class (say a subclass of Real) rather than as part of the core. I don't see this as generally applicable (e.g., in comparing the endpoints of intervals, or the real and imaginary parts of complex numbers, both of which are places where the tolType and tolerance values are used). I could see this as a special probability or percentage class, though.