Jahlani pushes her braids behind her ears, walking around the room. “Everything was put online. You had a whole weekend to look over it and ask questions.”
“Professor Jackson didn’t upload anything,” someone calls from the back.
Jahlani scratches her nose, moving to her laptop. “You sure?”
“There was nothing, Ms. Jones.”
She rolls her eyes. “It’s Jahlani. Just Jahlani,” she repeats under her breath as she clicks through the course landing page.
After the first few days, Jackson saw an influx of students. He came up with the idea for them to split the class in half. Every week, a set portion would move to a different room with her while the others would stay back with him. She didn’t expect to be teaching,teaching,but he has the type of authority that you don’t question, and she likes the challenge. Some deep, dark part of her wants to prove herself to him.
“It’s fine,” she announces, facing them. “We’ll just have to do double the work today.”
Moving to the whiteboard, she writes the equation.
“Okay, let’s look at the formula first. This is called posterior probability. All it means is this—” She continues speaking as she writes. “The probability of A happening given that B has already occurred. It’s an equation allowing one to find the probability of a cause given its effect. Are you with me so far?”
She turns toward the students, seeing a scattered wave of nods.
“Amazing. Let’s break this down further.”
Moving to the opposite side of the board, she sketches a box with arrows.
“These arrows”—she points to them—“represent conditional relationships.” She snaps the cover on the marker. “Okay, someone give me a pizza topping they hate.”
“Pineapple!”
“Okay, whoever said that, you’re not passing this semester.”
A few chuckles wave throughout the room and she feels warm at the reaction. “So, here’s an example of how this theorem works at the surface level. Let’s say you’re at a party. At this hypothetical party, you know that people who like pineapple on pizza are more likely to be from Florida—remember this ishypothetical,” she adds when a series of gasps and gagging sounds are heard across the room. She waits for everyone to settle.
“Okay, let’s change the example since we hate the pineapple one so much,” she says, as scattered applause starts.
“Let’s stick with pizza, but now it’s a deep dish. At this party, we know that people who like it are more likely to be from Chicago. So, the fact that someone is from Chicago makes it more likely they’ll like deep-dish style pizza.”
She pauses, waiting for questions. When none come, she presses on. “Now, if we know that an individual likes deep dish, we have to ask ourselves, how does that change our belief thatthey’re from Chicago? That is something that Bayes’ Theorem will help us calculate.”
Turning back to the board, Jahlani draws an arrow between the two factors, labeling them.
“So, P(A|B) is the probability that they arefromChicago, given that they like deep dish. And P(B|A) is the probability that someone from Chicagolikesdeep dish.
“So, maybe you start with some prior belief about deep dish preferences—say, sixty percent of people like it. But then, you learn someone’s from Chicago. Bayes’ Theorem lets youupdateyour belief about whether they like deep dish.”
She turns back and waits for students to write and type the information down. The door swings open, and she does a double-take. Her thoughts stutter for a moment because he’s here in another long-sleeve that is doing a lot for his biceps, and doing everything for her body. She lingers as she watches him move up the stairs swiftly, settling into the third row, second seat in. The same every time.
She’s suddenly irritated that she’s memorized where he sits.What a useless piece of information, she thinks, and turns away from his frame, shaking the very detailed images of what she thinks his arms look like underneath from her mind.
She clears her throat. “Sorry, what was I saying?”
Looking back at the board, she drops the marker, wiping clammy hands on her sides.
“You were telling us the purpose of Bayes’ Theorem,” a voice rings out near the front.
Right. Bayes’ Theorem.
She nods, picking up her water bottle and taking multiple sips. Setting it down, she moves back to the center, trying, and failing, not to look his way every three seconds.
The rules. Stick to the rules.
