Here's how the final course grade is calculated as a weighted average of the exam and lab scores.
A note about your lab scores, in case you notice your final lab score (when posted at the end of the semester) is higher than the one provided by your TA. I ask the TAs to give me a score out of 100. Since Excel provides several options for rounding decimal places up or down, I start with the score out of 100 which may include decimal places, and do all the rounding myself just for consistency across labs: I round UP no matter what. That means that if the lab score is 89.2 out of 100, it becomes 90 when rounded up. This may be what your TA does too, but I do it consistently for everyone. Lab is worth 35% of the course grade.
The three exams count for the other 65% of the course grade for a total of 100%, with exams 1 and 2 each worth 20% and exam 3 worth 25%. (The unequal weighting is due to the third exam playing the role of a quasi-cumulative final exam that isn't inflated by an optional extra points assignment as exam 1 and 2 are. But experience has shown that it makes little difference whether I weight them that way or just count all three exams equally, and in fact, if weighting the three exams equally -- i.e., 21.67% each -- results in a higher course grade for someone, I just give them that higher grade.)
The easiest way to see how these scores all get combined is to imagine a course total of 100 points, of which 20 come from exam 1, 20 from exam 2, 25 from exam 3, and 35 from lab.
Each exam has 40 questions on it. To make 40 exam questions add up to 20 points for each of exams 1 and 2, each exam question has to be worth .5 (or 20/40 = 1/2) points, so you multiply the exam 1 and 2 totals by .5.
To make exam 3's 40 exam questions add up to 25 points, each exam question has to be worth .625 (or 25/40 = 5/8) points, so you multiply the exam 3 total by .625.
To make the lab's 100 points count for 35 points toward the course grade, each lab point has to be worth .35 (or 35/100) points, so you multiply the lab score by .35.
So your course total is
.5(exam1) + .5(exam2) + .625(exam3) + .35(lab)
For example, scores of 34 on exam 1, 32 on exam 2, 24 on exam 3, and 90 in lab would give
.5(34) + .5(32) + .625(24) + .35(90) = 79.5
A letter grade cutoff is the lowest score required to earn that letter grade. Sample letter grade cutoffs for each exam and the lab are listed below. NOTE: These numbers are from another semester and do NOT represent the actual cutoffs that will be determined from this course's score distributions. They may be the same in some cases, but that is only due to the similarity of the exam score distributions, which can vary by semester for each exam. The LAB scores, however, use the same cutoff scores each semester. The final course letter grade categories come from applying the above weighting scheme to all the cutoffs. For instance, the course total required for a C from the cutoffs below is
.5(26) + .5(25) + .625(26) + .35(72) = 66.950
You earn the course letter grade corresponding to whatever category your course total falls into. The hypothetical student's score of 79.5 from the example above would earn a B, because it's higher than the B cutoff of 78.575 but not as high as the B+ cutoff of 83.225.
EXAMPLE NUMBERS ONLY - NOT NECESSARILY THIS COURSE'S ACTUAL CUTOFFS
MINIMUM SCORES (CUTOFFS) FOR EACH LETTER GRADE
EXAM 1 EXAM 2 EXAM 3 LAB TOTAL
max: 40 40 40 100 100.000
A 36 A 35 A 36 A 92 A 90.200 A
A- 35 A- 34 A- 35 A- 90 A- 87.875 A-
B+ 33 B+ 32 B+ 33 B+ 86 B+ 83.225 B+
B 31 B 30 B 31 B 82 B 78.575 B
B- 30 B- 28 B- 30 B- 80 B- 75.750 B-
C+ 28 C+ 27 C+ 28 C+ 76 C+ 71.600 C+
C 26 C 25 C 26 C 72 C 66.950 C
C- 25 C- 23 C- 25 C- 70 C- 64.125 C-
D+ 23 D+ 22 D+ 23 D+ 66 D+ 59.975 D+
D 21 D 20 D 20 D 62 D 54.700 D
D- 19 D- 17 D- 18 D- 60 D- 50.250 D-
F 0 F 0 F 0 F 0 F 0.000 F
Letter grade cutoffs for each exam are determined from the score distribution of each semester's class on that exam. That's why this semester's cutoffs cannot be known before the exams are actually given and the scores are analyzed.
Though a given semester's cutoff scores end up being fairly consistent from one semester to the next (due to consistent coverage of material and consistent class-level performance), different semesters may have lower or higher cutoffs than the example numbers shown above, for different exams. Lower cutoff scores generally result from more difficult exams. From one semester to another, the different cutoffs still reflect the same use of means and standard deviations, and the same criteria of fairness and generosity, as described below.
In general on my scale, the mean score divides the A and B grades from the C and D grades. Then roughly, the B grades are less than one standard deviation above the mean, while A grades are more than one standard deviation above the mean; likewise the C grades are less than one standard deviation below the mean, while D grades are more than one standard deviation below the mean, and F grades are more than two standard deviations below the mean. Pluses and minuses are added, as evenly spaced within each letter category as they can be. If those cutoffs end up being very borderline or ambiguous, I bend them in the direction of being generous, rather than making them stricter.
This scheme is always more generous than conventional percentage scoring (90% = A-, 80% = B-, 70% = C-, etc.), and in the unlikely event that a distribution was very high and this scale resulted in a lower score than the percentages would indicate, I'd give you the percentage-based higher score. In other words, you're guaranteed to get at least the score you'd expect from the conventional percentages, but my scale is usually more generous than that -- and it appropriately gives more of a boost to lower grades than to higher grades, since higher grades tend to already be nearly the highest they can get anyway.
I consider the common fix of a "curve" of adding a few points to every score (which mathematically speaking is actually a "line," anyway) to be arbitrary and ineffective, since the number of points added to the lower scores is limited by the number of points the instructor is willing to add to the highest score, when it's the lower scoring students who need the benefit of those points more. Surely the instructor adds points to allow more students to succeed at the low end, not because they think the highest scores deserve to be even higher. My scale helps more students and is the best I've come up with at being fair and generous with scoring standards. I hope you'll agree.