Conversion errors and the metric system


A recent headline on a metric system conversion error

I recently received news that the Centers for Disease Control and Prevention (CDC) made a major conversion error relating to the metric system. The upshot is that it greatly underestimated the risk of formaldehyde in laminate flooring. The underlying mistake: it failed to convert between meters and feet initially reporting the estimated risk at one-third of what it should have been.

The organization did not come out and say lack of metric adoption was the cause of the error:

The CDC/ATSDR indoor air model used an incorrect value for ceiling height.  As a result, the health risks were calculated using airborne concentration estimates about 3 times lower than they should have been.

However, others were more than happy to point out the real root of the problem:

CDC fixes major error in flooring risk report: Not converting to metric – Retraction watch

CDC Revises Health Risk Assessment Of Flooring After Math ErrorCDC recently announced that laminate floors are safe, only to realize that they forgot to convert from feet to meters—and that the cancer risk is three-fold higher –

There are some who believe that conversions are easy to make and therefore, living with two measurement systems shouldn’t be a problem.

Marciano's book

Marciano’s book

In fact, in his book, Whatever happened to the metric system: How America kept its feet, John Bemelmans Marciano (Kindle location 2020 for both quotes), states:

Conversion is now as easy as speaking “seven ounces to grams” into your smartphone and immediately receiving the answer 198.446662g.

Marciano later goes on to say:

Why would Americans go metric when computers have done the job for them and they don’t even have to know about it?

How about a three times greater risk for potential negative health effects due to human unit-confusion error?

Luckily, the CDC was able to quickly make a correction but who knows how many other errors haven’t been caught and continue to put us at risk in one way or another?

The idea that technology will save us from conversion errors is flawed because it assumes that the human element won’t impose the error.

Surely the CDC has access to computers and other high-tech gadgets at least as good, if not better, than what I have access to in my smartphone and yet, the mistake was still made.

Again, it’s not a technology issue, it’s a human issue that will always occur even if the frequency of such mistakes is not currently well known.

Aside from outright errors, there’s the time it takes to make a conversion in the first place. Add up the time it takes to whip out the cell phone, ask the question, wait for the answer and read it. Then one needs to multiply that by how many people in this country need to do that in a year. All wasted time. One set of measures eliminates the entire issue.

I’ve previously pointed out people are already at risk every time their pharmacist converts a prescription written in milliliters (as they all are) into teaspoons and tablespoons. Why are we doing this to ourselves?

Conversion errors are inevitable  

While I so far have been unable to find any statistics on how often conversion errors occur, everyone seems to recognize they do happen and research seeks interfaces that try to minimize them. One paper I reviewed, Reducing number entry errors: solving a widespread, serious problem by Thimbleby and Cairns indicates:

Ironically, the more skilled a user, the less attention they will pay to what ought to be routine outcomes, so the more likely these types of error will go unnoticed until they have untoward consequences. The reason is, as users become skilled, they automate actions, so their attention can be used more selectively; thus as they become more skilled, they pay less attention to the display, whose routine behaviour they have learnt to expect (Wickens & Hollands 2000).

A conversion "helper" from the 1970s

A conversion “helper” from the 1970s

Still, we can learn from our past. One of the things I’ve heard from people regarding our last attempt at metric adoption in the 1970s (I was a bit young at the time to remember) was students were taught difficult and confusing conversion formulas.

Next time, just have people start using the new, metric system measures and convert only those things that are absolutely necessary. Fewer conversions means fewer errors.

Thanks for reading.

More next month.


Medicine and the Metric System

Allow me to present my main point upfront: we are endangering our health by not adopting the metric system in this country.

Prescriptions are written in metric units. Conversions (and possible errors) are made at the pharmacy.

Doctors write prescriptions in metric units. Conversions (and potential errors) are made at the pharmacy.

Let me offer up a couple of examples that hopefully makes this clear. It’s important to understand that the medical field depends on metric units (as does most of science, for that matter). If healthcare workers talk in metric units and the public at large rely on U.S. customary units there is bound to be confusion and misunderstandings. That’s best avoided where your heath is concerned since the consequences could be dire.

Metric unit dosing is more precise

Last year Pediatrics (Official Journal of the American Academy of Pediatrics) published an article called “Unit of Measurement Used and Parent Medication Dosing Errors.”1 One of the article’s bottom lines:

Parents who used milliliter-only unit made fewer dosing errors than those who used teaspoon or tablespoon units. Moving to a milliliter-only standard could reduce confusion and decrease medication errors, especially for parents with low health literacy and non-English speakers.

While a minor mistake (whether too much or too little medication) might not make a huge difference for an otherwise healthy adult, these errors can be magnified for babies or those whose health is already compromised.

Most of our teaspoons and tablespoons are meant for eating, not dosing

No on should use "silverware" as substitutes for measuring teaspoons and tablespoons for medicine to avoid dosing errors.

No on should use “silverware” as substitutes for measuring teaspoons and tablespoons for medicine if they want to avoid dosing errors.

Another issue brought up in the piece was that the use of teaspoon and tablespoon employed for liquid medicines “may endorse kitchen spoon use.” I don’t know about you, but I have three sets of measuring spoons and many more spoons that I use for eating commonly referred to as “teaspoons” and “tablespoons.” The problem is, it’s the eating spoons that are often used to measure medicines. (Yes, I used to do that too, without even thinking about it.) If you have a dosing cup with only milliliters, the potential for confusion is greatly reduced.

As if that’s not bad enough

Even your actual measuring spoons aren’t as precise as you think they are. At one point I came across information indicating that up to a 20 percent variance is allowed. Again, that 20 percent could cause dosing errors. In researching this article I came across a page called “Cooking for Engineers” with the post:

I’ve got three sets of measuring spoons, and their measurements differ from each other, up to 1/4 teaspoon! Is there a way to know which (if any), are accurate?

The suggestions that followed involved scales and the temperature at which one should measure the water used to determine volume. Too bad no one suggested going metric.

Additional endorsement of the metric system for health reasons

I also have a document from the Institute for Safe Medication Practices (ISMP) called 2014-15 Targeted Medication Safety Best Practices for Hospitals2 . Two of its best practices mention sole use of metric system units.

Best Practice 3: Measure and express patient weights in metric units only.

The rationale:

Significant medication errors have occurred when the patients’ weight is documented in non-metric units of measure (e.g., pounds) and it has been confused with kilograms (or grams). Numerous mistakes have been reported when practitioners convert weights from one measurement system to another, or weigh a patient in pounds but accidently document the value as kilograms in the medical record, resulting in more than a two-fold error.

Best Practice 5: Purchase oral liquid dosing devices (oral syringes/cups/droppers) that only display the metric scale.

ISMP has received more than 50 reports of mix-ups between milliliter (mL) and household measures such as drops and teaspoonfuls, some leading to injuries requiring hospitalization.

Beware teaspoon and tablespoon instructions on prescriptions

Almost uniformly, prescriptions are written in metric units. However, if you pick up a liquid prescription and the dose on the bottle is not metric (and in reads teaspoon and tablespoons), the pharmacy has had to make a conversion. Where there are conversions, there is the potential for mistakes.

In addition, one of the top six recommendations in the 15th annual report of the National Coordinating Council for Medication Error Reporting and Prevention3 includes a “Statement of support for use of the metric system to dose medications.”

Advocate for the metric system and help make the country a healthy place!



Notes: The article itself requires a subscription However, a summary is located here: