$\DeclareMathOperator{\p}{Pr}$ $\DeclareMathOperator{\P}{Pr}$ $\DeclareMathOperator{\c}{^C}$ $\DeclareMathOperator{\or}{ or}$ $\DeclareMathOperator{\and}{ and}$ $\DeclareMathOperator{\var}{Var}$ $\DeclareMathOperator{\E}{E}$ $\DeclareMathOperator{\std}{Std}$ $\DeclareMathOperator{\Ber}{Bern}$ $\DeclareMathOperator{\Bin}{Bin}$ $\DeclareMathOperator{\Poi}{Poi}$ $\DeclareMathOperator{\Uni}{Uni}$ $\DeclareMathOperator{\Exp}{Exp}$ $\DeclareMathOperator{\N}{N}$ $\DeclareMathOperator{\R}{\mathbb{R}}$ $\newcommand{\d}{\, d}$

CS109 Midterm
Tue Feb 8th, 7pm. Location by last name

The Midterm and Solutions



Midterm


Solution


Your Exam

Do Review: If you missed some points, we encourage you to look over the answers and understand what went wrong. Make the midterm a learning experience and set yourself up for the final.

Regrade Requests: Humans graded your exam and they might have made a mistake! If they did, we would like to fix it. Please submit a regrade request on gradescope by Monday Feb 21st at 11:59p. Requests that often get points back: "grader didn't see my answer". Requests that often don't get points back: "The grader was accurate, but I think this approach deserves more points". We reserve the right to regrade the whole exam if you submit a regrade request.

Distribution

The median grade was an 110 out of 120 (amazing!), the mean was 107, and the standard deviation was an 13. Here is the distribution color coded with a way of interpreting your score:

Logistics

The CS109 midterm is a 2-hour, closed book, closed calculator/computer exam. You are, however, allowed to bring 10 pages (front and back) of notes in the exam, formatted in any way you like. Make sure to practice before the exam.

Location and Time

When: 7pm to 9pm Feb 8th
Where: We have reserved three rooms (Hewlett 200, Hewlett 201 and CEMEX). Location will be by last name.
CEMEX:
Last Name: A through K
Hewlett 200:
Last Name: L through S
Hewlett 201:
Last Name: T through Z

Covid Policies

It is an exceptionally intense time for Covid on campus. The university has suggested that cases are low enough that we are ready to meet in-person again. Having said that we are going to be accommodating to the situations that come up, whatever they may be, and we want to do everything in our power to make students feel as comfortable as possible. To that end we are not reserving one room (wtih 500 seats, as is standard) we are going to reserve three. This will greatly reduce the density of students in the exam room. We are going to answer FAQs here regarding covid as best we can:

Q: Should I take a covid test the week before the exam?
A: Yes

Q: I do not feel comfortable being in such a big room, what should I do?
A: We could imagine having covid anxiety for many reasons, especially if you are immunocompromised. If you dont feel comfortable taking the exam in one of the reduced density spaces, email us at cs109@cs.stanford.edu before Wednesday morning and we can make alterative arrangement for you.

Q: What if I feel sick the day of the exam?
A: Don't come to the exam. Send us email and we will figure something out.

Q: I am in covid isolation, what do I do?
A: Email us and we will make arrangements with you.

Alternative Arrangements

If you have an academic conflict with the midterm time (say you also have a CS999 exam at the same time) you should let us know right away (before thursday Feb 3rd at 9a). Email cs109@cs.stanford.edu. Similarly if you have special university accommodations please contact us before Thursday to help us schedule.

SCPD Monitors

If you are an SCPD student you will need an exam monitor. You should make arrangements to take the exam within 24 hours after 7p on Tuesday. The exam monitor may be remote.

Coverage

The midterm puts special emphasis on the material from the first three problem sets and the first four sections. This includes material in lecture up to and including class on Wednesday Feb 2nd. In the reader this corresponds to Part 1 and Part 2, and Part 3 up through the "inference" section.

Answer Format

You are going to be solving probability questions by hand. To that extent we are not interested in numberic answers, but rather in formulaic answers. It is fine for your answers to include summations, products, factorials, exponentials, and combinations, unless the question specifically asks for a numeric quantity or closed form. Where numeric answers are required, the use of fractions is fine. You must show your work. Any explanation you provide of how you obtained your answer can potentially allow us to give you partial credit for a problem. For example, describe the distributions and parameter values you used, where appropriate.

What about the Phi table? I am not going to make you look up values from a phi table. Instead you can leave your answer in terms of phi (the CDF of the standard normal). For example $\Phi(\frac{3}{4})$ is a fine final answer. This was not the case in the past so you will see questions which ask for a numeric answer in the practice exams.

Essential Practice

Extra Practice

Note: You should not expect that a TA will know these problems in office hours (there are far too many for them to "prep" them all)

Review Sessions

One of our wonderful TAs is making a recorded review session! It will be posted by Sunday morning Pacific Time at the latest.

You can do it!