# Quiz Bowl: SI vs Conventional Units

Welcome back, quiz bowl participants. Our very first quiz bowl, which appeared on this site on May 5, 2011, dealt with format, style, and punctuation of units of measure. Now we’re back to delve even further into the intriguing and ever-changing world of units of measure style. The following is a sample of one of the questions that appears in this month’s AMA Manual of Style quiz on SI vs conventional units (http://www.amamanualofstyle.com/). Answer the question based on your understanding of section 18.5 of the AMA Manual of Style.

The mean 2-dimensional area of the largest metastasis was 7.2 sq in.

So, what do you think? This one isn’t really too hard. Here’s the answer (use your mouse to highlight the text box):

The mean 2-dimensional area of the largest metastasis was 46.8 cm2.

Measurements of length, area, volume, and mass are reported in metric units rather than English units. To convert square inches to square centimeters, multiply by 6.5 (§18.5.1, Length, Area, Volume, Mass, pp 794-795 in print).

Let’s try a more challenging question.

Admission laboratory tests revealed the following: serum creatinine, 0.9 mg/dL; serum urea nitrogen, 11 mg/dL; serum albumin, 39 g/L; and prothrombin time, 11.5 seconds.

Yes, this one is definitely harder. Here’s the answer:

Admission laboratory tests revealed the following: serum creatinine, 0.9 mg/dL (to convert to micromoles per liter, multiply by 88.4); serum urea nitrogen, 11 mg/dL (to convert to millimoles per liter, multiply by 0.357); serum albumin, 3.9 g/dL (to convert to grams per liter, multiply by 10); and prothrombin time, 11.5 seconds.

For laboratory values, factors for converting conventional units to SI units should be provided in the article. In text, the conversion factor should be given once, at first mention of the laboratory value, in parentheses following the conventional unit (§18.5.10, Laboratory Values, pp 797-816 in print). Although the AMA Manual of Style recommends writing out units of measure when no numbers are reported (eg, micromoles per liter), some journals may prefer to use abbreviations when listing SI conversion factors (eg, mmol/L).

Need some more practice? Subscribe to the AMA Manual of Style online and take the full quiz. If you’re new to the site, check out some of our other quizzes as well.—Laura King, MA, ELS

## One thought on “Quiz Bowl: SI vs Conventional Units”

1. M R Page

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.