The percent error calculation formula is a way of measuring to analyze the closeness between measured value and actual value. Where the measured value is compared to a true or exact value. The percent error is calculated by dividing the difference of the measured value and the exact value by the exact value and multiplying it with 100. The formula is as below –

Percent Error = [(Measured Value - Exact Value) / Exact Value] x 100

The percent error can be positive or negative, depending on whether the measured value is larger or smaller than the exact value. A smaller percent error result indicates a better measurement. It means if the per cent error value is close to 0, then your measurement is more perfect.

Percent error calculation is important to know the accuracy of measurement. Accuracy means the degree of closeness of a measured value to its original or true value.

For example, if you measure the length of a table as 121 cm, but the actual length is 120 cm, then the percent error is:

Percent Error = [(121 - 120) / 120] x 100 = [1 / 120] x 100 = 0.00833 x 100 = 0.833 %

So, the percent error is 0.833%.

Percent error calculation is an important part of dispensing pharmacy to ensure proper drug dispensing or dose adjustment.

For example, if you need to weigh 750 mg of a drug, but you actually weigh 748 mg. The percent error is:

Percent Error = [(748 - 750) / 750] x 100 = [-2 / 750] x 100 = -0.00266 x 100 = -0.266%

Another example,

If you need to calculate the dose of a drug for a child using the formula: Dose (mg) = (Weight (kg) x Adult Dose (mg)) / 150. The adult dose is 200 mg and the child’s weight is 15 kg. The correct dose is:

Dose (mg) = (15 x 200) / 150 Dose (mg) = 20

However, you mistakenly use the formula: Dose (mg) = Weight (kg) x Adult Dose (mg). The wrong dose is:

Dose (mg) = 20 x 200 Dose (mg) = 4000

The percent error is:

Percent Error = [(4000 - 20) / 20] x 100 = 1990%

This means that your calculation is 1990% more than the correct dose.

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