Artist's depiction of an exoplanetary system, SSC Astronomy 141:
Life in the Universe
Prof. Richard Pogge, MTWThF 12:30

How I Compute Your Course Grade:
A Worked Example

Please Read This First!

This example uses representative data from the Astronomy 161 class I taught Winter Quarter 2004, and is meant as a worked illustration of the grading process I follow for all of my introductory classes (Ast 161, 162, and 141).

Please keep in mind, however, that this is an example - every class uses the same basic procedure, but are each a little different in detail (e.g., number of exams/homework, relative weighting, etc.). Trying to using the data here to estimate or predict your grade on your own will usually lead to an erroneus or misleading result.

I compute your overall course grade by converting the scores on the exams and the homework into a grade-point score on the OSU 4-point grading scale, and then combining these grade-point scores to compute the final grade and generate a letter grade.

The divisions between A, B, C, etc. for each of the grade curves define the "break points" of the conversion. Note that this is not a simple percentage conversion (for example, I do not use 90%=A, 80%=B and so forth), nor is the conversion between a test score and a grade-point score a simple formula (I have to do a numerical interpolation on the break points, but that's a computational detail).

Homeworks are slightly different in that I first add up all of the scores for the 5 homeworks, and then create a curve for the total to use to convert total score into a grade-point equivalent. Because there are only 5 questions, it is pointless to try to curve each homework assignment.

The 4-point grading scale is defined in terms of the lowest grade-point score (GP) that is assigned to each letter grade:

       GP   Letter
      3.85    A     
      3.50    A-  
      3.15    B+    
      2.85    B     
      2.50    B-
      2.15    C+   
      1.85    C     
      1.50    C-    
      1.15    D+    
      0.50    D 
The way to read this table is as follows: in order for person to get a B+, their grade-point score must be at least bigger than 3.15, and below 3.50. Note that OSU does not give D- or F grades, but does give grade-point scores below 0.50 an "E".

To illustrate how a grade is computed in detail, I've created a fictitious student, Claude Ptolemy, who got the following test scores this quarter. The grade-point scores were computed using the grade curves for the various exams and the cumulative homework assignments.

	      raw     %
	     score  score   GP  Letter
    Exam1    23/50   46%   0.50   D
    Exam2    31/50   62%   1.93   C
    Exam3    29/50   58%   1.64   C-
    Exam4    32/50   64%   2.07   C
    Homework 18/25   72%   2.81   B-
    Final    72/100  72%   2.71   B-

In computing Claude's overall grade, I do the following:

  1. I drop the worst of his four in-class exam scores. For Claude this is Exam 1 where he got a grade-point score of 0.50 (D). The remaining three in-class exams will each count for 15% of the final grade, or 45% of the overall grade taken together.

  2. If there are extra credit points, they are applied to the score on the Final Exam. Thus, for example, if Claude had gone to the planetarium program and collected the extra credit point, I would have used a corrected Final exam score of (72+1)=73. Mr. Ptolemy didn't attend, so I will use the base score of 72.

  3. I sum the scores from the 5 homework assignments (5 assignments of 5 questions each) and then curve that cumulative score to generate the homework component of the grade. The homework counts for 15% of the overall grade. Here, Claude got 18 of the 25 homework questions right, which comes out to a grade-point equivalent of 2.81.

  4. I curve the final exam separately from the in-class exams and homework, and convert it into a grade-point equivalent. The final counts for 40% of the overall grade. Here, Claude got 72/100 on the final, which is equivalent to a grade-point score of 2.71.

Once this is done, the final grade is computed by combining each of the three pieces. For Claude, this looks like this:

 Overall = 0.15*Exam2 + 0.15*Exam3 + 0.15*Exam4 + 0.15*Homework + 0.4*Final
         = (0.15*1.93) + (0.15*1.64) + (0.15*2.07) + (0.15*2.81) + (0.4*2.71)
         = 0.290 + 0.246 + 0.311 + 0.422 + 1.084
         = 2.352
In this example Claude has an overall grade-point score of 2.35 for the class. On the OSU 4-point scale, the lowest grade-point score that gets a C+ is 2.15, and the lowest score that gets a B- is 2.50. Since 2.35 is between 2.15 and 2.50, Claude will get a grade of C+ for the class.

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Updated: 2012 February 9
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