Chapter 3: Estimation (sanity checking)

There is a problem in the scenario below. What is the problem?

A patient is prescribed 3,000 micrograms (3,000 µg) of morphine over 24 hours. A nurse quickly calculates that the patient thus requires 12.5 µg per hour.

The calculation is clearly incorrect. To see this, round the 24 to 20, just to get a very approximate result using a calculation that is easy to do mentally: 3,000 ÷ 20 is equivalent to 300 ÷ 2, which is 150. So, we would expect an answer in the region of 150 µg. This suggests that the result 12.5 µg is wrong, and the calculation should be done again. The correct result is 125 µg. This is a potentially serious error in dosage!

This example highlights a very common problem. That is, when performing even very simple calculations, it is very easy to make an arithmetic error. The source of the error might be simply transcribing the number incorrectly, inverting a function (dividing instead of multiplying) or pressing the wrong button on your calculator. One of the best ways to check a calculation is to first estimate your answer.

Share This Book