Chapter 279: Thresholds and Boundaries

The cursor blinked in the terminal like a second hand. The segment of spike-and-slow waves from 03:17 was highlighted, amplitude 48 microvolts, mixed in among the burrs of respiratory artifact and EMG interference. Lin Chen stared at it, his finger hovering over the Enter key. Treat it as a training sample, or preserve it only as a read-only appendix? If he wrote it into the core parameters, the model’s recall rate for early warning could be pulled up to 0.79, but its specificity would fall below the investors’ red line of 0.85, and it would cross the ethics boundary. If he excluded it completely, the data would be clean and the defense would go smoothly, but what he missed might be a real precursor seizure. Clinical practice was not a math problem. There was no absolute optimal solution, only risk preference. He closed his eyes and thought of the stars Xiaoman had traced over and over in crayon in his picture book. There were no miracles in reality, only traces that had been confirmed again and again. He entered the command: appendix_spike_0317 = True, then added a note beside it: not_for_training_or_validation. He was not gambling on probability; he was leaving evidence without crossing the line.

The shadow-evaluation script began running. The progress bar crept upward, and the cooling fan gave off a low hum. From the vending machine at the end of the corridor came the occasional crisp clink of a falling coin. A night-shift nurse pushed a treatment cart past, its rubber wheels rolling over the terrazzo floor with a regular rustle. Lin Chen propped his left foot on the empty chair beside him. The ice pack had already melted, and the water seeping from it had spread into a dark patch on his trouser leg. The cramp spread from his ankle to his calf, the muscle twitching beyond his control. He bit his lower lip and made no sound, only pulling the keyboard a little closer into his lap. Lines of log output swept across the screen: [INFO] Loading readonly appendix anchor... [INFO] Running shadow evaluation... [WARNING] Specificity if promoted: 0.86 -> 0.842. He opened the confusion matrix and marked only the sensitive interval of the classification boundary in the defense appendix, without overwriting the production threshold. He could not force the numbers to fit, and he could not quietly turn a hospital read-only segment into a training sample. He created a new file, risk_disclosure.md, and wrote the items one by one: "1. The added segment is a single-case anomaly and has not reached statistical significance; 2. It is only a defense appendix and clinical review prompt and does not enter the training set or validation set; 3. The model output indicates only a risk level and does not replace clinical diagnosis; 4. Long-term video EEG review is recommended." What the investors wanted was certainty. What the hospital wanted was exemption from liability. What he provided was a boundary.

At 2:40 in the morning, the corridor lights went completely dark, leaving only the green emergency exit sign glowing faintly. His mother pushed the door open and came out holding an enamel mug filled with warm water. She did not turn on the main light, only used the faint glow of the screen to set the cup on the corner of the desk. “Drink it while it’s warm.” Her voice was kept very low, for fear of waking Xiaoman, who had just fallen asleep. Lin Chen gave a quiet “Mm” without looking up, his fingers still typing. His mother stood there for a while, her gaze passing over his swollen ankle and the dense code on the screen. She did not ask whether it hurt, nor whether he was tired. She simply reached down, pulled up the thin blanket that had slipped to the floor, and covered his knees. “Don’t push yourself too hard.” She turned and went back into the ward, and the plastic curtain fell shut softly behind her. Lin Chen lifted the enamel mug. The warmth of the water slid down his esophagus, pressing down some of the hollow chill in his stomach. He continued changing parameters. He adjusted the number of cross-validation folds from 5 to 10 and ran the grid search again, but marked the output directory separately as appendix_only. Computing power was tight; the laptop keyboard was so hot it numbed his fingertips. He did not dare use external power, afraid of tripping the breaker, so he could only hold out on the battery. The charge dropped from 42% to 19%. He closed every background process, leaving only the evaluation script. The screen dimmed by one level. He lowered the brightness and kept watching.

At 3:15, the shadow evaluation finished. The terminal displayed the appendix results: Sensitivity: 0.784 | Specificity: 0.841 | AUC: 0.867. A little short, but it explained the boundary. He exported the review note, the read-only session screenshot hashes, and the risk disclosure file, packaging them into the v2.1_appendix_only directory, while the production weights remained on the original version. Then he opened the server log audit tool. The due diligence team would arrive tomorrow afternoon to examine data provenance, access records, and the model iteration trail. He archived every parameter adjustment, every hash value from every data cleaning pass, and every email exchange related to ethics filings from the past three months along a timeline. The filenames strictly followed the YYYYMMDD_HHMMSS_action.log format. There were no shortcuts, only dogged, painstaking work. He checked the permission settings three times, confirming that all sensitive fields had been de-identified and all external interfaces had been closed. When that was done, he leaned back in the chair and let out a long breath. Outside the window, the sky was beginning to turn gray. In the distance, an early bus rolled over the wet asphalt, giving off a dull rumble.

At exactly four o’clock, his phone vibrated. It was not an email, but a WeChat message from Su Man: "The due diligence team has changed its schedule. Legal and audit are going straight to the company at eight tomorrow morning. Zhao Qiming is leading the team himself. Can you sync the physical access control for the server room and the cold backup data on your end?" Lin Chen stared at the screen, his pupils contracting slightly. They had moved it up by twelve hours. He replied quickly: "Yes. I’ll be at the server room before six to complete the final snapshot and permission handoff. Have the compliance checklist ready." After sending the message, he closed the laptop and stuffed the USB drive and the paper archive folder into his backpack. When his left foot touched the floor, the sharp pain stabbed into his nerves like a needle. He steadied himself against the wall, hopped two steps on one leg, and adjusted his balance. The first thread of dawn filtered through the window at the end of the corridor, falling across the fire evacuation diagram on the wall. He opened the ward door and glanced at Xiaoman sleeping soundly and his mother dozing against the headboard, then gently closed the door again. The elevator descended, the numbers changing one by one. He knew that today’s battle had only just begun.