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Partial Credit on Checkbox Questions
Google Forms allows for checkbox style questions such as these:
And the responses to these questions get put in the Google Spreadsheet like this:
Since Flubaroo does a simple comparison of student response to answer key, a student's response will be either completely correct (the student checked the exact same boxes), or completely incorrect (they did anything else). This isn't always what you want.
How to Grade Checkbox Questions with %cb:
But it is possible to tell Flubaroo to give a student partial credit for checking some of the right boxes. To do this, just prepend the special %cb modifier before your answer key, like so:
In this case, Flubaroo will give points for boxes checked correctly. And to ensure that a student can't just check every box to get 100% on a question, Flubaroo will also remove points for boxed checked incorrectly (with a minimum score of 0 points).
Example #1 (Ralph): Say I assigned 1 points to this question (1/3 point for each correct box checked). In Ralph's submission, he only checked one correct answer ("Conservation of Energy"), and two incorrect answers. As such, his total score will be: 1/3  1/3  1/3 => 0 points.
Example #2 (Lisa): Say I assigned 3 points to this question (1 point for each correct box checked). In her submission, Lisa checked all the correct boxes. But she also checked an incorrect box ("Conservation of Velocity"). As such, her total score will be: 1 + 1 + 1  1 = 2 points.
IMPORTANT: Google Forms places a comma between each student's answer, and Flubaroo uses this comma to determine which boxes the student checked. This will cause a problem when the choices in the form themselves contain a comma. So avoid commas in your checkbox question choices.
Changing the Behavior for Incorrect Choices:
By default, Flubaroo will subtract points for incorrect choices, as demonstrated in the example above. Each incorrect choice is worth the same number of points as a correct choice. But you can change this by placing a numeric multiplier immediately following %cb. Below are two examples:
In both the examples below, a checkbox question worth 3 points has 3 possible correct choices (so 1 point for each correct choice, and 1 point for each incorrect choice):
Example #1: %cb0: The '0' following %cb tells Flubaroo to multiply the incorrect point value by 0, making incorrect choices worth 0 points, and thus effectively removing the penalty for incorrect answers.
Example #2: %cb0.5: The '0.5' following %cb tells Flubaroo to multiply the incorrect
point value by 0.5, making incorrect choices worth 0.5 points, rather than 1 points.
Note: The multiplier can be .25 or any number you wish.
Here is a video that explains the process that Flubaroo uses for gradin with %cb:How the %cb operator works. 
