Tale Behind 98.6 Fahrenheit

Did you know the tale behind why the “normal” body temperature is considered to be 98.6˚F? Tim Harford tells it in his book, How to Make the World Add Up.

 

More than a century back, a German doctor named Carl Wunderlich assembled over a million readings from 25,000 patients. Over a period of 18 years. The average of those million readings became the de facto answer. However:

“Wunderlich’s numbers were off; we’re normally a little cooler (by about a Fahrenheit degree).”

How could he have gotten it wrong despite that many readings? Well, for one, his thermometers were never calibrated. Another reason was that he took readings via the arm pit.

 

But the biggest reason why the number 98.6˚F has stuck is what I found the most interesting.

 

Wunderlich’s readings were in centigrade, and his average value was 37˚C. The implicit margin of error was 1 degree (the accuracy of his thermometer), i.e., the band between 36.5 to 37.5˚C. He wrote his articles describing his results in German (He was German, remember?). Then his articles got translated into English, and as part of that the number was converted to Fahrenheit and thus became 98.6˚F. But wait:

“(Such a precise number in conversion) invited physicians to assume that the temperature had been measured to one tenth of a degree Fahrenheit rather than one degree centigrade.”

See what had happened? The implied precision just changed by a factor of 20 from one degree centigrade to one tenth of a degree Fahrenheit! Not by malice or deceit but because of a conversion followed by assumptions. A million readings, (wrong) assumptions about the precision of the thermometer had combined to inspire extreme trust in the number.

 

And so, like the (deliberately) inefficient QWERTY keyboard on our laptops, the “wrong” normal temperature of 98.6˚F stays the accepted value of our normal body temperature.