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Tackling number problems

When faced with number problems at work, it is important not to launch straight into calculating. Your chances of success will be greatly improved if you work through the problem in a step-by-step fashion. 

Use the following method for 'PEACE' of mind! 

Step 1: Plan - what are you trying to do?

Before you begin, consider the type of problem you are trying to solve. What are you looking for? What will the solution tell you? For example, do you want to know how many tablets to give or, how much medicine a patient requires throughout the day? It is also very important to consider what’s at stake and your comfort level with what you are doing (are you familiar with the drugs, the condition, the patient)?

In the following situations, particular care needs to be taken:

  • when you are working with unfamiliar drugs or patient groups
  • when you are working with powerful medicines
  • when administering medicines to children or infants
  • when administering medicines to the very sick or vulnerable.

When you are at the 'Plan' step it is important to consider what is at stake. When the stakes are higher you need to take even more care than usual and take the necessary steps to ensure you calculate correctly. When the stakes are high it is particularly important to get help when you are unsure or, have others check your work.

It is also important to consider how accurate you need to be. In some cases absolute accuracy is not useful or practical. For example, fluid balance charts for adults may record urine outputs measured using jugs marked only at 50ml intervals and do not need a higher degree of accuracy however, when nursing children, smaller measurement intervals may need to be applied.

Step 2: Estimate - what does a sensible answer look like?

Before you calculate it is important to roughly work out what a sensible answer might be. This is a very important step as it makes it easy to catch simple mistakes (for example, inputting the wrong number into a calculator) and gives you another way to check your answer.

There are lots of different ways to estimate an answer before calculating. You will find some different methods for estimating the answer to number problems in the 'Estimation' section of this resource. It is very important to note that estimation is not a substitute for properly performing a calculation. It is there as an aid to getting the correct answer and giving you confidence in the value returned by your calculation.

It is important to note that the only way to ensure patients receive the correct amount of medications is to perform the calculation correctly or seek help and advice if you are unsure how to do this. Drug calculations are NEVER estimated.

Step 3: Approach - how are you going to solve the problem? 

You are now getting close to the step where you can actually do the calculation. However before you do this, there are still some important decisions to consider. Ask yourself the following questions before going any further.

What - what method are you going to use?

Can you use mental arithmetic (e.g. adding up and down to get the answer or looking at relationships between numbers)? Or, do you need to apply a formula? In this resource for the different situations presented, we will always show you a formula you can use.

How - do I carry out the steps that are involved?

It is often easier to break a big calculation into smaller steps. For example, you might first convert units to be the same before applying a formula or, you might first need to calculate the weight of a patient. You should also consider if there are tools to that can help you. You may find there are graphs, charts, or ‘ready reckoners’ that can help.

Graphs and charts can make it easier to understand and interpret complex information. An observation chart shows not only the last recorded temperature, it also reveals a trend which may help diagnosis and save time looking through written notes. Conversion tables are useful and time saving when conversion of one set of measurements to another is frequently required.

Another tool is the calculator however, there are things you need to think about when using a calculator:

  • The calculator can only respond to the numbers you put in it - if you make a mistake entering numbers the answer you get out will be wrong (hence the need to estimate first).
  • You need to be familiar with the calculator - it is particularly important to know the order in which it performs calculations.

Step 4: Calculate

When performing the calculation, there are some simple things to remember:

  • check each step
  • take your time
  • do not feel rushed/pressured
  • take care when using a calculator.

Step 5: Evaluate your answer

After performing the calculation you need to check it. Ask yourself. 'Does this answer look right? Does it fit with the estimated answer I came up with in my estimation?' If it doesn't, then you are probably right in thinking that the calculation is wrong. Some possible ways you can check your work include:

  • repeat the calculation
  • ask a colleague to check your answer
  • try to calculate the answer again using a different method
  • check against the recommended dose range (e.g. using the British National Formulary)
  • look for unusually big or small answers
  • check against your experience - is the value in the range of what you are used to?

It is always useful to ask a colleague to double-check your calculation if you feel even the slightest uneasiness about your result. After all, any of us can make a simple error on a particular day. In your workplace there may even be a policy that every calculation must always be double-checked when dispensing medicines. Don't be tempted to skip this step - it will prevent errors happening that cause potentially serious harm to patients.

Harm to patients from calculation errors is a primary concern but there are other consequences too. 

Page last updated - 26/02/2024