They decided to split up the investigation between the two of them.
Mikoshiba went around talking to the people at the shop. Who decided the orders, when, and how. The sequence of the prep work. How the craftsmen's day flowed. What kind of man Tadokoro, who had died, had been. —The gritty stuff that never turned into numbers, he gathered up with a smile, without a trace of reluctance. It was a feat Mio could never in her life have imitated.
Mio, meanwhile, borrowed the back office and poured three years of shipping ledgers and inventory time series into her laptop, single-mindedly.
The idea itself wasn't difficult. Hidden in a sequence of past numbers were repeating "waves." Waves by day of the week. Waves at the beginning and end of the month. Seasonal waves. Find them and add them together, and you could roughly read "when, and how much, it will sell next." Reading inventory like an EKG.
Around dusk, Mikoshiba came back to the office. The smell of the steam had soaked into his jacket.
"How's it going? Seeing anything?"
"In progress. And you?"
"They're interesting people," Mikoshiba said, pulling a folding chair over and sitting down. "This Tadokoro. Apparently he'd been the owner's partner for thirty years. Quiet, stubborn, but he turned into a different person when it came to prep. A craftsman whose mood for the day showed on his face depending on how the tofu turned out."
"There's no record of a value called mood here."
"I suppose not." For some reason, Mikoshiba laughed a little at her curtness. "But how about this. You heard what the veteran worker said. That Tadokoro went outside every morning without fail and looked up at the sky."
"I heard," Mio said, not taking her eyes off the screen. "That he adjusted the prep by the weather and the cold."
"Right. Without teaching anyone, all in his head. —Does that have anything to do with your work?"
"I don't know."
Mio answered honestly. The habit of looking up at the sky. It wasn't written anywhere in the shipping ledgers. Mio had stored that remark somewhere separate, and still hadn't grasped where she ought to connect it.
For now, it was the waveform in front of her. Mio dove back into the screen.
At the start of the following week, Mio had a hunch.
She tried lining up every stockout day from the past three years, one after another. When she did, the stockouts seemed to cluster on certain "days of the week." Specifically, during particular weeks of the month—roughly the second through the third week—the scooped tofu sold out mostly on Tuesdays and Fridays.
"...I have a lead."
Mio built a model that weighted the orders and prep more heavily on those days. When she fit it to the past three years of data, it matched as beautifully as anything could. The day-of-the-week wave, the wave of position within the month. Added together, they explained nearly all the past stockouts.
"Agreement with past data, nearly perfect. This will prevent the stockouts."
For once, Mio felt a slight sense of a solid result. This would be the end of it, she thought. Too soon, a warning sounded in a corner of her mind, but the goodness of the fit silenced it.
But that sense of a result slipped away with no resistance at all.
The week after they arranged a trial prep schedule following the model, Ohata Tofu Shop ran out of stock again. On the Tuesday that Mio's forecast said to "weight heavily," the scooped tofu piled up and was thrown out, and on the Wednesday about which it had said nothing, the scooped tofu sold out early and went out of stock. The forecast lurched out of line with reality.
"...It doesn't fit."
Mio muttered it low, still glaring at the screen.
"It fit the past perfectly."
Impossible, she thought. Three years of stockouts had been explained that beautifully by nothing more than the day of the week and position within the month. And yet, when she actually had it forecast the next week, it was practically useless.
Mio inspected the model's internals line by line. She tried lowering the Tuesday coefficient. She tried cutting the weight on the third week. No matter how many times she rearranged it, it clung perfectly to the peaks of the past, yet next week's troughs alone slipped right through her fingers.
"...This isn't a parameter-tuning problem."
Mio set the days of the week aside for the moment. The remark Mikoshiba had gathered was snagged in a corner of her mind. That Tadokoro looked up at the sky every morning and adjusted the prep by that day's weather and cold.
Then, the sky. She lined up three years of weather and temperature again, beside the shipments. First, weather. Sunny, cloudy, rainy—did the way the scooped tofu sold change with that day's sky? Next, temperature. How many degrees it had been that day.
Neither could fully explain the peaks of stockouts.
A gentle overall lift, maybe, she could just about see. In the height of the heat, the scooped tofu for chilled tofu sold better across the board. There was certainly that much of a tilt. But that wasn't what Mio wanted to catch.
The problem always came suddenly.
In the waveform from the ledgers, here and there across gentle flatlands, were spikes that thrust straight up. For one day only, demand would leap, the prep couldn't keep up, and the scooped tofu would sell out. The stockouts always happened at those spikes. And yet neither the day of the week, nor the weather, nor that day's temperature showed anything at all special on the day before a spike. A calm, sunny, ordinary day. And the next day, it suddenly jumped.
"...None of the three reaches the sudden peaks."
Mio let her body sink back into the chair. She looked up at the ceiling and didn't move for a while. The days of the week only traced the past and missed the future peaks; weather and temperature didn't even point to where the peaks were in the first place. The more she worked at it, the better the fit to the past became, and the further she got from the crucial jumps. She knew that feeling.
—A trap I stepped in again and again at my last job.
"...I'm overfitting."
Mio covered her face with both hands and glared at the screen once more through her fingers.
Mikoshiba, back from the floor, stood behind her and peered at her profile.
"It's not the day of the week, not the weather, not the temperature. These peaks that rise up all of a sudden can't be explained by any of that."
Mio let it slip out low, not really addressed to Mikoshiba.
"So—what you just said, Himuro, broken down in terms I can understand?"
Mio hesitated a little. She wasn't good at putting her own misreadings into words. She pressed a fingertip to her temple, once.
"...The stockouts always happen on days when demand suddenly jumps. And I still can't pinpoint those days. The day of the week just carved its shape to match the past. It fits the past perfectly, but it misses tomorrow's peaks. I tried weather, and temperature too. But sunny or rainy, how many degrees that day was—that 'state of the day' itself didn't connect to the jumps."
"It hits the past, but it doesn't hit tomorrow's sudden peaks."
"Right. Not the day of the week, not the weather, not the temperature. Whatever it is that pushes demand up for a single day is still hiding somewhere else."
Mio traced the line graph on the screen again. There was certainly a pattern to the way it was disturbed. And yet she couldn't grasp the thing itself that was producing that pattern.
The data wasn't lying. Every one of the three years of numbers was accurate. And yet it didn't fit. The bad texture of it lingered, gritty, on her fingertips.
"...Eerie," Mio murmured, almost unconsciously. "The correct data is all here, and it isn't enough. Something was never in these ledgers to begin with."