main-midtermgrading

How to interpret your midterm grade

Note: we will not assign letters grades to the scores until students have had the opportunity to “rebut” their scores. Since scores may change, the distribution may change, and therefore the grade assignments.

Your scores are in a file in your handin directory. The answer key is pub:midterm-key.pdf.

The midterm had 70 points, the grades ranged from 34-64. The mean and median were 50 and 51 respectively.

Because of issues on questions (described below), we corrected grades. We gave students who lost points on Questions 6 and 13 one point (unless they already got full credit). The corrected mean and median were both 52, and the standard deviation was 8 points.

You can retrieve your exam either during the professor’s office hours, or at the end of class on Monday, November 12th. If you take your exam, you must return it to the professor before the beginning of class on Friday, November 16th. If you think your exam was graded incorrectly, you should send email to the professor explaining what you think was wrong before Friday, November 16th.

Note: if you ask for your exam to be regraded, we will re-examine your entire exam, not just the questions you think were graded incorrectly. Also, if we regrade your question 6 or 13, we may not give you the correction points.

Some comments on interpretting the grades:

Question 6

Many students misinterpretted the meaning of “only see A” (thinking that this meant you could see B). If it was clear from the work that you showed that you misinterpreted things this way, we graded the question to see if you got the right answer to the question that you interpretted.

We did mark off -1 point for misinterpetation, but gave +1 to everyone (who didn’t receive full credit). Therefore, you were not penalized for this misinterpretation.

The most common mistake was to forget that the results of the compositing operation were pre-multiplied, so your final answer was double pre-multiplied.

Also, some people forgot to give an equation for the resulting alpha.

Question 7

• Z - miss 1 or 2 numbers (probably arithmetic error) -1pt
• Y - get edges wrong (-1 or -3 if miss both)
• X - wrong number of numbers (-2)
• W - shifted answer (-2)

Question 9

We marked the common failures people made using the following codes:

• X - you used a negative scale (not allowed in the directions) -2 pts
• Y - translate by other than 0,1 -2 pts
• X - didn’t reflect the B -2 pts
• W - didn’t put the last B in the right place -2pts
• T - type (we think you meant the right thing, but didn’t write it)

As far as length:

• 16 lines or less - full score
• 17-19 lines = -1pt
• 20 or more = -2pts

Question 13

We lumped common wrong answers into 4 categories:
WS - you assumed that the test procedure was too simple (like the initial image only tested against a white background). (WS means White or Simple)
BL - you had some answer about the inability to see blue
YG - you had some answer that said that yellow and green would be confused or indistinguishable (the real answer is that yellow is not green)
WD - “what difference” - this answer meant we couldn’t understand why what you described in your answer made a difference

Because so many people got this question wrong, we are willing to attribute part of the problem to a bad question (i guess we never were specific that real bananas are really yellow), so we gave a correction factor.

This question had been used on previous exams, and we hadn’t had problems with it in the past.