knows that repeated division is equivalent to dividing by a product? Far more likely, then, the nurse would have calculated 4× 24 on paper or used the calculator to store the result in the calculator’s memory. He or she would then need to do AC AC 5250 ÷ 45. 57 ÷ MRC = to get the answer.

How can one work out 4× 24 and store it in the memory? A basic calculator has a memory, but many calculators do not have a “store in memory” key; instead, they have an “add to memory” key, M+. To store a number to memory, then, the memory must already be zero, otherwise the number stored will be wrong. If the nurse starts to calculate 4× 24 before zeroing the memory, it is almost impossible to store the result correctly.

To get the drug calculation right, the nurse must do the following: AC AC MRC MRC 4 × 24 M+ 5250 ÷ 45. 57 ÷ MRC =. The buttons AC and MRC must both be pressed at least twice at the start, otherwise the nurse risks the wrong answer being calculated.

In computer science terms, what the nurse has just done is called “compiling”; the nurse has converted, that is, compiled, a calculation into a sequence of “machine code operations”— button presses—to do it. To compile correctly, which is crucial to get the right answer, the semantics of the target machine (the calculator) must be defined; but we know many calculators are very different (and, worse, mathematically wrong) despite even looking alike [ 5]. Clearly, compiling is a difficult task for any user, and indeed one can imagine it is especially difficult for people

trained as nurses rather than as computer scientists. We do not know from the incident investigation whether the nurses got the compiling wrong, were using the wrong numbers, or “simply” missed a step in their calculation. But requiring nurses to do such a complex operation as compiling for such badly specified devices as handheld calculators is manifestly risky. The cognitive load on the user will not have helped their vigilance and ability to detect errors. Whatever the causes, we do know they made an unfortunate calculation error they did not detect.

Although compiling on a calculator is very complex, it is still generally easier than doing the calculation with pencil and paper. One of the main reasons compiling is so complex is that calculators are designed to do any calculation—they are more powerful than any nurse or doctor needs. If the nurse makes a mistake, perhaps pressing – instead of ÷, no calculator will complain; it has no idea what calculation the nurse is trying to do. It will just provide the wrong answer. If the design of calculators is inappropriate for medical calculations, it is even more remarkable that the infusion pump did not help, as it—unlike a calculator—was specialized to medical problems. Its design should have been based on a task analysis and potential user errors. An infusion pump contains microprocessors, and one could easily be designed to take concentra- tion, duration, and so on from the nurse and do the sum itself. Some are, of course, but not this one (the ones that do, so-called “smart pumps,” cost much more

despite differing only in their programming).

Interactive Medical Devices
Are Bad Too

In the 2006 fatality, both nurses failed to divide by 24 hours per day, so they agreed the dose rate was 28. 8 mL per hour instead of the correct 1. 2 mL per hour. The pump could have told the nurse that at that rate the drug supply (which the pump knows) would take about four hours to be used. This would not have been the four days the nurse expected. Instead, the infusion pump merely asked the nurse to confirm what they had entered. They had entered 28. 8 in error, so the pump asked if they had meant to enter 28. 8. Unfortunately, having made the error, that was what they thought they wanted to enter.

Since the pump was in use
exclusively on a chemotherapy

[ 5] Thimbleby, H. “Calculators are Needlessly Bad,” International Journal of Human-Computer Studies 52, no. 6 (2000):1031– 1069.

 Figure 1. The infusion pump used in the fluorouracil incident. The pump is small and gives the patient full mobility during the treatment. Here, the nurse needs to enter mL per hour but has to use Option 3, which is apparently asking for mL! Note that the up arrow key doubles as the decimal point.

September + October 2008