Chapter 20
Ivy’s head was pounding. Her tongue thick and rubbery.
Something was beeping.
Heart monitor, probably. She’d drunk too much and had been admitted to the hospital. Stomach pumped. IV drip installed.
No . . . she’d gone home with Mr. Finance. He’d drugged her. Him and his buddies had run a train on her. Dropped her off at the side of the road, clinging to life.
Ivy knew that neither of these were true, but she couldn’t stop her hands from roaming all over her body. Gently prodding, probing.
Nope, none of that happened, and the beeping wasn’t a medical device.
It was her fucking phone.
She reached for it, knocked it off the bedside table. Groaned. Rolled over. Grabbed it. Hammered blindly at the screen.
It stopped making noise.
“I hate you, Abs.”
She squinted at the screen. There was a single text message from her friend. She frowned, deleted it. Then Ivy closed her eyes.
Wait.
She had work today.
Fuck.
Ivy grabbed her phone again, opened one eye just wide enough to see the time. Paper width.
It was after eight.
Shit.
Ivy sat up. Waited for the world to stop spinning. Gagged. Swallowed. Waited some more.
When vomiting all over herself—her outfit from last night—transitioned from a certainty to a mere possibility, Ivy finally got out of bed. New data, Bayesian statistics at its fnest.
She had to get moving, had to get to work.
Ivy stumbled to the bathroom, grabbed a face cloth. Wet it and scrubbed.
One of the worst inventions in the history of mankind was waterproof makeup. In theory, it was a great idea. In practice, not so much. It was a pain in the ass to get off.
Sure, there were makeup removal chemicals, a secondary industry created from the first, but Ivy didn’t have the fortitude nor the time to start rooting through her drawers.
She rubbed until her skin turned red.
Her hair was a mess. Abby had straightened it yesterday, but it had battled to return to its native curly state overnight.
Gave up somewhere halfway. It was a wavy, kinky disaster.
Ivy brushed her teeth, scooped water in her palms and drank.
Cold—so cold.
Feeling marginally better now, she quickly slipped out of her dress and into her ‘uniform.’ Tied her hair back.
Got the fuck to work.
“Sorry I’m late.” During the short drive from her house to campus, most of the events from last night came back to her. Blake. Tony.
Flipping quarters for drinks—Penney’s game.
Zeke . . .
She’d hoped that he’d slept in, too—he’d been lit at the bar. But Zeke was already in class when she arrived.
What annoyed Ivy more than the entitled kid’s presence was his appearance. Sporting a sharp, crisp polo, his blond hair perfectly coiffed.
Fuck him.
“Tristan, can you pull up the graphs on Bayesian statistics again?”
Her TA, who hadn’t mentioned her appearance and had a cup of hot coffee waiting for her, obliged.
Ivy turned around, glanced at the digital display. The two bell curves, one pink, one blue.
She sighed. Touched her forehead. It was burning up.
She needed a different approach.
What did Einstein famously say?
The definition of insanity is doing the same thing over and over again and expecting a different result.
Something like that.
Ivy lowered her head, closed her eyes. Opened them again and glanced over at Tristan sitting behind his desk. He stared back, a confused expression on his face. Silently urged her to get started.
“Okay, okay.”
There was a podium at the front of the class, but Ivy rarely used it. It was too formal. She intended on using it now, however. With the stylus, she pressed the clear button on the embedded digital pad.
“I’m going to try something a little different today. We’re going to do a deep dive into Bayes’s Theorem, which is the basis of Bayesian statistics.”
I hope you’re right, Tristan.
A couple of groans.
Sex, alcohol, and drugs.
“Let’s say that the probability of a first-year student contracting an STI is 5 percent.”
The groans stopped. Replaced by a solitary uncomfortable chuckle. Ivy, worried that her resolve would fade, kept her eyes locked on the screen. She scribbled: “STI probability = 5%.”
Her penmanship was horrible.
“Now, let’s say you were a bit . . . concerned after a particularly eventful weekend.
You decide to get tested. The test isn’t perfect—they rarely are.
Now, if you have an STI, the test is fairly accurate—95 percent of the time, it will come back positive.
But if you don’t actually have an STI, the false positive rate is 10 percent. ”
She wrote these numbers on the board, too.
No laughs now; no groans, either.
A good sign? Were they finally following along? Or just too shocked to react?
Ivy still couldn’t believe that she was going through with this. The lasting effects of alcohol were still lowering her inhibitions.
Had to be.
“Hypothetically, after your weekend, the test comes back positive. What are the chances that you have an STI?”
Ivy finally raised her eyes. She was surprised to see everyone’s attention locked on the display. Everyone except Zeke, because Ivy still couldn’t bring herself to look at him.
“Anyone?”
“95 percent?” someone offered.
And that’s why you guys bombed the first test.
“Not quite. Don’t freak out just yet. Here’s Bayes’s formula.”
P(A∣B)= P(B∣A)?P(A)
P(B∣～A)?P(～A)+ P(B∣A)?P(A)
“Where P(A) is equal to the probability that someone in first year has an STI—so 5 percent. P(B|A) is the opposite, that you don’t have an STI—95 percent.
P(~A) is the true positive rate of the test—also 95 percent.
While P(B|~A) is the false positive rate—people who don’t have an STI but still test positive—10 percent. ”
Ivy was on a roll now, writing at a rapid clip. Penmanship still awful.
“Now, to find the actual probability that you have an STI and tested positive, all we have to do is run the numbers.”
The math was fairly simple, and Ivy had no problem performing the calculations even with her foggy brain. She wrote out all the steps, then circled the final value.
≈0.333.
“So? What’s the answer?”
A beat of silence.
Ivy was worried that she’d lost them somewhere along the way. Not sure how, given how simple her example was. Simple and relatable.
Then someone said, “It means that Zeke might not have gonorrhea!”
The class erupted into laughter.
Try as she might, Ivy found it impossible to keep from smiling.
“It means that you should always double bag it in first year!”
“Yeah, I think the horse has already left that barn on that one,” Ivy said.
More laughter.
“Okay, calm down. Calm down. The actual chance that you have an STD? 33 percent repeating.”
There it was. The light bulbs. Bright, shining, illuminating their young, eager, horny faces.
For fuck’s sake, Tristan was right. You just had to speak on their level.
Now Ivy glanced to the upper left-hand corner of the class. Saw Zeke.
His face was red, bordering on purple.
The smile on Ivy’s lips grew.
Good—fuck him. He’s an asshole.
Do you know who my father is?