amastyleinsider

We love comments. (From real people, that is. Spambots, you can stop any time.)

We love comments like, “Great blog!” We even love comments like, “You are wrong about every single thing related to medical editing, your mother was a hamster and your father smelled of elderberries, goodbye.” Both of those require simple responses—I like “Thanks!” for both, to be honest. For the latter, I would be charitable and not even comment on the comma usage.

Every once in a while someone will leave a critical comment that requires a longer response, and sometimes the consultation of outside experts. That was the case with this Quiz Bowl post on units of measure. A reader wrote:

A big problem with the AMA manual is a lack of consideration significant figures. The conversion factor listed in the online “SI Conversion Tables” section from feet to centimeters is 30. That’s wrong. Let’s say I try to convert my height (6.0000 feet) into centimeters. The “.0000″ means that my measurement has 5 significant figures. Significant figures are important in science and health care.

I start with the only unit conversion between customary and metric that matters: 2.54 centimeters equals exactly 1 inch. This is the only conversion that matters because it is a definition. There are infinite significant figures.

Here is what happens if I use the “SI Conversion Tables” section of the AMA manual of style:

6.0000 feet * 30 = 180 centimeters

Here is what happens if I use math and pay attention to significant figures:

6.0000 feet * (12 inches/1 foot) * (2.54 cm)/(1 inch) = 182.88

Where did those extra 2.88 centimeters come from? They came from a a conversion factor that was wrong.

For the same reason as above, your answer to the first problem is wrong.

7.2 inches^2 * (((2.54 cm)^2)/((1 in)^2)) = 46.45 (assuming 4 significant figures, to demonstrate the inaccuracy of your conversion factor)

This isn’t just an academic exercise. A text for editors shouldn’t have errors like this.

We made “hmmm” noises for a while but finally drafted a response to post here, since a shameful amount of time has gone by since the original comment.

You raise an important point about the significance of significant digits. The Manual addresses this in section 20.8.1 and, in chapter 18, where the conversion table is embedded that shows conversions for inches to centimeters, there is a caution that results should not be reported beyond the appropriate level of precision.  It is critical to ascertain the precision needed for the clinical context of the conversion. If you only need significance to 1 place beyond the decimal (7.2 inches) to accurately describe tumor size, then the 2 significant digits of the result should be fine and the clinical difference between 46.8 and 46.5 is probably not important.

It’s entirely possible that the final 3 words of that paragraph are the equivalent of a thrown gauntlet to someone out there—if so, we’re willing to continue the conversation in the comments to this post.—Brenda Gregoline, ELS