Penalty mechanism

What it is for

When the quiz is run in adaptive mode the student can interact with a question repeatedly. So in particular the student can try again when he gets a wrong answer. Clearly the final mark for the question must reflect the fact that the student did not get it right originally. Therefore a penalty is subtracted from the final mark.

How the penalty is determined

First of all penalties are relevant only if a quiz is run in adaptive mode. Only in this case can a student have a second attempt and therefore only in this mode can there be any occasion to subtract a penalty.

Even in adaptive mode the penalty mechanism is only used when it is selected in the quiz options. If "Apply penalties" is set to "No" then the final mark for the question is the mark for the last graded response.

Each question has a 'penalty' field (which should really be called 'penaltyfactor') which is a number between 0 and 1. The penalty for a wrong response is calculated as the product ($quiz->penalty * $quiz->grade), i.e., as the product of the penaltyfactor with the maximum achievable grade for the question. This product is stored in $state->penalty. So $quiz->penalty is the fraction of the maximum grade that is subtracted as a penalty for each wrong response.

The $quiz->penalty field has a default value of 0.1, both in the database and in mod/quiz/defaults.php. This default can of course be overwritten by the admin on the quiz configuration page. This admin-selected default is (as usual for admin defaults) stored in $CFG->quiz_penalty. The teacher can choose a different penalty factor for each individual question when adding or editing a question.

Now if a student makes repeated wrong attempts (or partially correct attempts) the penalties for all these attempts are added up in $state->sumpenalties. The mark for the question is then calculated as the mark for the last graded response minus the sum of the penalties.

One curious fact about $state->sumpenalties is that, for efficiency reasons, it is not stored in the quiz_states table but instead in the 'sumpenalty' field of the quiz_newest_states table. That way it only has to be stored once per attempt rather than once per response.

Where it is done in the code

The function quiz_apply_penalty_and_timelimit() subtracts the penalty in $state->sumpenalty from the raw grade in $state->raw_grade to obtain $state->grade for the response. However it is ensured that the grade of a new attempt at the question never falls below the previously achieved grade. This function also increases $state->sumpenalty by the amount in $state->penalty. The assumption is that $state->penalty has just been set appropriately by the code calling this function, e.g., quiz_process_responses.

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