### Bar Graph on Quiz Results - Grade Ranges

Number of replies: 140-1, 1-1, 1-2, 2-2, 2-3, etc. If I increase the maximum grade, say to 12 (2 points per question), on the editing quiz tab, the bar graph then shows the grade ranges as 0-1, 1-2, 2-3, 3-4.

Does anyone know why this is? The second display is the one that I want and I don't understand why the first one is as it is.

I would be grateful for help asap on this one. Thank you!

### Re: Bar Graph on Quiz Results - Grade Ranges

http://cvs.moodle.org/moodle/mod/quiz/report/overview/overviewgraph.php?view=markup&pathrev=MOODLE_19_STABLE

The bit just after the comment

//pick a sensible number of bands depending on quiz maximum grade. I guess it is not

being all that sensible. Perhaps we need a better algorithm.

### Re: Bar Graph on Quiz Results - Grade Ranges

### Re: Bar Graph on Quiz Results - Grade Ranges

Hi Tim

Do you know of any future plans to "*find a better algorithm*" to determine the bands in the quiz results graph?

We have quizzes with a maximum grade of 10, zero decimal places in grades and see 0-1, 1-2, 2-3, 3-4, etc as the range.

This is not helpful as it's not clear whether this contains the results for '1' or '2' or both.

Cheers

Teresa

### Re: Bar Graph on Quiz Results - Grade Ranges

Would you like to suggest an algorithm?

Or even just suggest what the bands should be for your specific case.

### Re: Bar Graph on Quiz Results - Grade Ranges

Call me a purist, but for me this graph is not a band graph it's an histogram and bands are in fact bins.

According to wikipedia (see above link Number of bins and width) *There is no "best" number of bins, and different bin sizes can reveal different features of the data* so finding a best algorithm is not an easy task. Wikipedia page suggest several formulas.

I too find the label of bins quite confusing.

The algorithm is in fact using the formula bin = floor(grade/binwidth) so is following the "common use" and for instance "1 - 2" means that 1 is included but 2 is excluded. The last bin is a special case because the upper limit is also included because it's the quiz max grade.

As a side note, some countries use the exact inverse rule: they include the bigger value and exclude the smaller for each bin except the first one .

Last remark: looking at this (ugly) graph remind me of Joseph's message in the developer's forum about including a better graph library in Moodle

### Re: Bar Graph on Quiz Results - Grade Ranges

There may not be a theoretical 'best' number of bins, in an abstract sense, but we have a specific purposes here: we want to automatically draw a histogram of these results of this quiz as a 600px-wide image to display in a web browser (or print). And while we don't need a 'best' set of bins, we need a choice that is good. And then we need a way to display that axis so it is not confusing. That is our problem, and I think it is well-enough defined.

### Re: Bar Graph on Quiz Results - Grade Ranges

Jean-Michel you appear to be quoting Wikipedia as a reference on a site used extensively by academic. Is this wise

### Re: Bar Graph on Quiz Results - Grade Ranges

@Marcus,

I am of opinion that is quite OK to quote Wikipedia on a forum. Things would be quite different in a research paper, a thesis, etc. of course, if only because Wikipedia articles do not have authors.

Joseph

### Re: Bar Graph on Quiz Results - Grade Ranges

Hi Tim

I've asked the staff member who queried this what they think they'd like but have not had a reply. I cannot suggest an algorithm but I believe the main point he was making was that it is unclear if a student's score of '2' would be included in the '1 - 2' band or the '2 - 3' band or both...

Would it be possible to have them appear as "0.00 - 0.99, 1.00 - 1.99, 1.99 - 2.00 etc" so it is clear. I also think if there are less than 10 then "1, 2, 3, 4, etc" would work too.

Cheers

Teresa

### Re: Bar Graph on Quiz Results - Grade Ranges

The problem with 0.00 - 0.99 and 1.00 - 1.99 is that it is then not clear where a score of 0.995 would go. That would be displayed as 1.00 when rounded to 2 decimal places, but would acutally go in the '0.00 - 0.99' bin.

An accurate way to label the axis is something like 0 ≤ < 1, but that strikes my overly mathematically pedantic.

### Re: Bar Graph on Quiz Results - Grade Ranges

Or [0 ; 1[ but that's is not better .

### Re: Bar Graph on Quiz Results - Grade Ranges

Hi Tim

Here is the response from the staff member I was working with regarding this - I hope it helps:

"0, 1, 2, 3, etc" would be ideal,

otherwise "0.00 - 0.99, 1.00 - 1.99, etc" as you suggest

or perhaps "0.00 - 0.49, 0.50 - 1.49, etc".

The number of bars should by M+1 where M is the maximum

grade for the quiz, or (M+1)/K where K is the smallest

integer such that (M+1)/K is less or equal to the maximum

number of bars they want to show.

### Re: Bar Graph on Quiz Results - Grade Ranges

Thanks for asking about this.

I think the key thing there is the +1.

I don't think the K thing is quite right. I think that we alwasys want the 'step size' to be something like ..., 0.1, 0.2, 0.5, 1.0, 2.0, 5.0, 10.0, 20.0, .... - and we chose the value there that gives us the number of bars we want, say between 7 and 15.

Acutally, I once wrote some very nice code that did that. I wonder where that was? If I can find it, I will reuse it. Otherwise, I will have to re-code it.

### Re: Bar Graph on Quiz Results - Grade Ranges

Since it is showing "grade ranges", shouldn't it be tied to the letter grades set up, so you can see the distribution across that?

If you have 93% as an A, then the number of students with percentages 93-100 would be shown in the A grade range. 90-92% in the A- grade range, etc...