rounding LTI passback grades

[ophelia-adams]

It appears that WeBWorK rounds the scores sent via LTI to an LMS to the nearest integer.

This happens in line 175 of lib/WeBWorK/Authen/LTIAdvanced/SubmitGrade.pm:
$score = wwRound(2, $score);

While this appears consistent with how rounding is done elsewhere (cf #513) those instances of rounding are for display, rather than calculation. It seems to me that, for numbers which will be used in grade calculations, exactness is preferable, so that rounding occurs just one time when calculating and displaying grades to students through the LMS.

I believe LTI 1.3 allows sending exact scores (separate score & total score) but I am not familiar enough with the compatibility considerations involved in WW's choice to only report percentages. In any case, it seems like sending as many significant figures as the LTI standards allow would suffice. (I acknowledge that even the current rounding scheme almost surely has no effect on student grades, but I can see how it might set off some worry)

[dlglin]

The code you quoted only applies to LTI 1.1. The code for sending grades via LTI 1.3 is in lib/WeBWorK/Authen/LTIAdvantage/SubmitGrade.pm, and already sends the exact score and total score.

It looks like the LTI 1.1 code has always had this rounding. I don't know if there is a reason that it was included.

[ophelia-adams]

I see -- I believe my institution is using 2.15, which probably precedes LTI 1.3. The wiki documentation seemed to contradict that info about LTI 1.3. Thanks! Might still be advisable to remove rounding from the LTI 1.1 version.

[kyleclaassen]

I'd like to request that this issue be reopened. I'm still using LTI 1.1 for grade passback, and rounding the score ratio to two decimal places generates obvious discrepancies in the LMS (Moodle for me). For example, suppose a set has 6 problems and the assignment in the LMS is set to be worth 6 points. If a student scores 5 out of 6 in WeBWorK, then "0.83" is passed back to the LMS, which shows as 4.98 out of 6 instead of 5 out of 6. Naturally, this generates student complaints.

I'm not sure why the rounding was implemented, but it doesn't seem to be required by the LTI 1.1 specificiation; see Section 6.1.1: replaceResult in the LTI 1.1 implementation guide. The example XML message uses a value of "0.92" (indeed two decimal places), but rounding isn't mentioned anywhere. I've been commenting out the wwRound line, and the grade passback to Moodle still works.

I'd like to propose adding a configuration variable $LTI{v1p1}{enable_score_rounding} in conf/authen_LTI_1_1.conf with a default value of 1 for backward compatibility:

# Enable or disable rounding of problem set scores for grade passback.
# If 0, then do not round problem set scores.
# If 1, then round score to two decimal places. E.g. a score of "5 out of 6" would be passed
# back as "0.83".
# For safe backward compatibility, the default value is 1 since WeBWorK has rounded to two
# decimal places historically.
# $LTI{v1p1}{enable_score_rounding} = 1;

Then the round in lib/WeBWorK/Authen/LTIAdvanced/SubmitGrade.pm could be replaced with

if ($ce->{LTI}{v1p1}{enable_score_rounding} // 1) {
    $score = wwRound(2, $score);
}

I suspect that $LTI{v1p1}{enable_score_rounding} = 0 is what most (if not all) LTI 1.1 users would want, but note that this code would safely default to the historical behavior of performing the round if a configuration migration/upgrade happens to omit the new variable.

[dlglin]

@kyleclaassen this was implemented in #3019.