# 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.*

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.