This morning I ran across the following tip from Joost Helberg on Mastodon:
TIL don’t report numbers with three digits after the decimal point. People may interpret the decimal point as a thousands separator. Using 2 or 4 digits, although wrong, avoids off by a factor thousand errors.
I usually report four decimal places, but I hadn’t thought about that in relation to the decimal separator problem.
In software development, it’s best to let a library handle numeric input and output, using local conventions. Failure to do so can lead to problems, as I’ll never forget.
Embarrassed in Bordeaux
In 2006, Peter Thall and I gave a week-long course on Bayesian clinical trial design in Bordeaux, France. Part of the course was presenting software that my team at MD Anderson Cancer Center had developed.
A few minutes into my first presentation I realized the software wasn’t working for the course attendees. The problem had to do with the US and France using opposite conventions for decimal separator and thousands separator. I had tested our software on a French version of Windows, but I had entered integers in my testing and decimals in my presentation.
I apologized and asked my French audience to enter decimals in the American style, such as 3.14 rather than 3,14. But that didn’t work either!
We were using a Windows API for parsing input, which correctly handles input and output per local conventions. But we had written custom validation code. We checked that the input fields contained only valid numeric characters, i.e. digits and periods. Oops!
Users were between a rock and hard place. The input validation would not accept French notation, and the parsing code would not accept American notation.
The solution was for the attendees to set their operating system locale to the US. They were gracious about having to apply the hack and said that it was a common problem. It was a humiliating way to start the course, but the rest of the week went well.
I learned about this in the early 1990s when reading through the manual for the (then-new) HP10C calculator:
“A radix mark is the divider between the integer and fractional portions of a number. A digit separator distinguishes the groups of digits in a large number. In some countries the radix is a decimal point and the digit separator is a comma, while in other countries the reverse is true.”
The function is toggled by holding the period down while turning on the calculator.