It can happen that the answer model that you've set up in an exercise has an error in it that you overlooked when creating the exercise. This is particularly troublesome when students have already taken a test with this exercise in it and some of them got their answers scored erroneously. One option is to regrade all these answers manually, but we also have a built in option to regrade exercises automatically for this very case.
Do not change or add any variables
The first thing you need to do is correct the answer model of the exercise. It is however important that you do not change or add any variables.
Variables are generated when a student takes an exam. From there on, they will never change again. So if you add a variable or change the definition of a variable after the test was generated, this will not be picked up by the exercises that were generated before your change and regrading based on these new variable definitions will not work.
You can, however, add solution and feedback rules and change existing ones, as long as they do not require extra variables.
After you've corrected the exercise, you can choose to regrade it. Go to Teacher -> tab Tests and click the icon in the Results column. From here, go to the Result per question tab and click the Regrade button in the row of the exercise that you want to Regrade. After this, the regrading will start and you will be directed back to the Results per student tab, where you will see which tests are currently regrading. This view will refresh every 5 seconds, until the regrading of all tests is finished. Regrading might take a while, so please be patient.
Only submitted tests will be regraded.
If you already manually graded some answers, then these grades will not be overwritten by the regrading.