How many marks should I take off for a student using an incorrect value? I am marking tests and a student wrote down 64 instead of -64 for one of the questions. This obviously changed the entire question and they would have gotten full marks otherwise, (i.e. all their steps are correct). They also did really well on the rest of the test. I do not know how much I should penalize them since full marks on this question would give them 87%, and anything lower would be an &lt;85% mark. Even half marks would drop their mark to as low as 60%. How many marks would you penalize them?
Marks are a measurement of how well the student knows the subject. So it depends on how much of a lack of knowledge that mistake represents:

in most cases, if all steps are correct and only a number was copied wrongly, then obviously the knowledge of the subject is very good, and most of the question was done correctly, so &quot;almost full marks&quot; are deserved

if the mistake shows lack of knowledge (for example, if that &quot;-64&quot; is a value of power, which can only be positive, or if it leads to an obviously unphysical result that the student should have noticed), then one might remove more marks.

a tricky case is if the mistake leads to the problem being much easier to solve (e. g. &quot;Write the last 10 digits of 2^(64+x)&quot;), and it is correctly solved. In this case, it would be unfair to give full marks, as the student did not show much knowledge of the subject. Unfortunately, this should probably result in no marks.


It seems like your case falls in the first category, so I would see no reason to give anything less than &quot;the highest mark that is not a full mark&quot;.