Calculating Percentage by P_Gallo


A percentage tells you ‘how many out of 100’.
So for example, if 20 of your 200 patients with diabetes have not had an annual review, you would
know that 10 out of every 100, or 10% are not being reviewed.
Percentages are useful if you want to compare proportions of different totals: for example, you may
know one year that 200 of your 2560 patients are over 75 years old, but if next year you have 210 over
75s and your total list size has changed to 2769, you would need a percentage to tell you whether the
proportion of over 75s had increased or decreased.

                         To find X as a percentage of Y:

                 Divide X by Y (X is usually smaller than Y)
                             Multiply by 100

For example:

        You want to know what percentage of your 115 patients who did not turn up for appointments
        last week were children. 22 were aged under 16.
        To find the percentage of under 16s:
                          22 / 115 x 100 = 19% (rounded down from 19.13043%)

Increases or decreases as a percentage

        If you want to express an increase or decrease from an original number as a percentage, you
        need to add an extra step.

                   To find the percentage increase or decrease

      Find the difference between the original and the new number
              Divide the difference by the original number
                             Multiply by 100

For example:

        Your practice immunised approximately 200 children with the MMR vaccine each year from
        1992 to 1997 between January and June. In 1998, you immunised only 150.

                 The difference between 200 and 150 = 50
                 50 / 200 x 100 = 25%

        You therefore have a 25% decrease in your immunisations this year, compared to the previous
        five years.

NB Use a calculator if possible: it’s much easier

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