Fable
Two doctors, one prescribes by complexion, the other counts coffin records
There were two doctors in town. Doctor Wu walked around town every day, observing the neighbors' complexion, asking about their appetite, and listening to their coughs, and whenever he felt someone was unwell, he would visit and prescribe medicine. Doctor Gu kept the town's death records in his cabinet—how many people died each month, from what causes, and at what ages—and organized them into charts and tables in his clinic.
In the first year, Dr. Wu ran around every day and adjusted prescriptions daily, and few people in town fell seriously ill. Dr. Gu's death record book was quite beautiful—low numbers, stable trends. Everyone said the two doctors had different approaches but both showed the town's health was fine.
In the middle of the second year, a problem arose. One day Dr. Wu found seven neighbors suffering the same ailment at once—a dry cough with blood, and a sharp drop in appetite. That same day he wrote a collective prescription and distributed it, and notified the town so everyone could drink preventive decoction. The numbers in Dr. Gu's death record book for that month were still very low—no one had died yet, so naturally there was no record. But if Dr. Wu had not found it early, those neighbors with dry coughs might two months later have become the statistics in Dr. Gu's next month's count.
At year-end the town clinic did an evaluation. Dr. Wu's metric was called health score—how many had sallow complexions this month, how many had reduced appetite, how many improved after drinking the preventive decoction. Dr. Gu's metric was called mortality rate—how many died this month, up or down from last month. Placed side by side, the health score began dropping in May, while the mortality rate only rose in August. A full three months apart.
The clinic director drew a relationship diagram. The health score is a leading indicator, using daily observations and behavioral signals to detect problems early. The mortality rate is a lagging indicator, using events that have already occurred to infer trends in reverse. The health score can tell you it is time to write a prescription; the mortality rate can only tell you the prescription was written too late.
One question stumped the curator: at what health score should an alert be triggered? Doctor Wu said everyone's constitution is different, and the same symptom in a frail person should trigger a warning two weeks earlier. But two weeks earlier would mean prescribing many more preventive formulas, which is quite wasteful. Doctor Gu said death records are still the most accurate—if a person dies, they die; the signal is unambiguous. The curator gave the conclusion: death records confirm the trend, health scores determine when to act.
Finally, a line was hung at the entrance of the town clinic: a low health score is telling you it is time to farm; a rising mortality rate is telling you this year's harvest is already gone. You need to look at both, but you cannot wait until harvest calculation time to remember farming.