The numerical grade you receive on each test is a raw score. It has nothing to do with percentage correct. Nor does percentage correct have anything to do with letter grades. For each test I establish a correspondence between the raw score and percentile ranking within the class. I use this to establish boundaries between the letter grades. Normally the A/B boundary is set at roughly 80th percentile and the B/C at 40th, i.e., 20% of the class is in the A range and 40% in the B.
To average together the grades from tests with different raw scores I map each test onto a 100 point scale. The highest grade in the class becomes a 100; the A/B boundary is set to 90; B/C to 80; C/D to 70; D/F to 60. Between the boundaries I use linear interpolation. For example, on a quiz where the top grade was 132, the A/B boundary was 122, and the B/C boundary was 110, a 123 would map into 91.00, a 121 would map to 89.17, a 113 to 82.50, etc. The the numbers, e.g., 90, are entirely arbitrary; they are not a percentage of anything.
The letter grades on the papers will mapped onto a similar scale: A = 95; A- = 93; A/B = 90; B+ = 87, etc. (I give strange grades like A/B.)
I make a weighted average of all of the grades and use the numerical average as a guide to assign the final grades. (I do give +'s and -'s.) If I have evidence for other factors, perhaps exceptionally hard work, or lackadaisical work, I will take this into account. I aim for a final grade distribution somewhat higher than that of the tests, typically 70--80% A's & B's. I adjust a bit depending on my opinion of how the current class stacks up against others in recent years.
At the time of the exam you will be able to chose a weight from 10--40%. If the weight is less than 20% your high quiz will be weighted upward accordingly. If the weight is greater than 20% the low quiz will be weighted downward. By chosing the exam weight to be 40% your lowest quiz will disappear.
Since 2003 the average course grade distribution has been
It is not my intent to give any D's or F's. The students who got F's worked really hard to demonstrate that an F was the grade they deserved.