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For comparisons / evaluations of objects of the same class(and other purposes), is it better to define a static or a non-static (is this called "dynamic" by chance?) function?

Sample code:

class student
{
    internal decimal medianOfMarks()
    {
        int count = 0;
        decimal sum = 0;
        foreach(float mark in reportCard)
        {
            counter++;
            sum += mark;
        }
        return sum / count;
    }

    **OR**

    internal static decimal medianOfMark(student delinquent)
    {
        [same logic]
    }
}

Let's not consider making the function a property, though parameterless, since I'd like an answer applicable to parameterized functions as well.

What are arguments pro / con each approach?
What deciding factors do exist?
I've already thought into this myself, but I don't want to bias the discussion with my opinion.

Mark
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    See here http://programmers.stackexchange.com/questions/199735/static-or-non-static – blgt Jun 16 '14 at 09:51
  • @blgt that one is more about performance, memory consumption in particular. For me this is more about which one is better style, better comprehensible and good practice.
    I am by far not concerned about running out of memory.     – Mark Jun 16 '14 at 11:30
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    Do you need access to private members? Yes? Make it a method. No? Make it static. – Doval Jun 16 '14 at 12:18
  • @Doval that's the kind of distinction-helpers I'm looking for. – Mark Jun 16 '14 at 12:38
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    If you're not concerned about what's under the hood, than @Doval's comment is the alpha and the omega. static == no state, non-static == state. – blgt Jun 16 '14 at 13:18
  • Your function is called median but computes the arithmetic mean/average. – CodesInChaos Jun 16 '14 at 14:47
  • I'd make this a method of ReportCard. If you need this on the student, you should forward the call to ReportCard. 2) You should use a generic averaging method which takes a sequence of numbers and outputs a number. Such as Enumerable.LINQ 3) I wouldn't use float on the report card. Either go with decimal or with integers representing fixed point numbers.
    • – CodesInChaos Jun 16 '14 at 14:54
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    I always put static members in a separate section of the source code. This makes it clear that they are completely different from non-static members. Microsoft code analysis automatically warns you if a method could be static, but is not. – Frank Hileman Jun 16 '14 at 17:25