# How to Calculate Precision

Precision measures the repeatability of an event or sampling error. Take five shots at a basket and record the location each ball strike. If the ball hits near the same spot on the backboard, but misses the basket, its strikes are precise, just not accurate. Use standard deviation to measure precision.

1. ## Determine Average, or Mean

• 1

Count the number, or N, of data values.

• 2

• 3

Divide the sum by N (from Step 1) to determine the average.

## Determine Variance

• 4

Subtract the average from each data value to calculate each value's deviation.

• 5

Square each deviation.

• 6

• 7

Subtract one from the count, or N - 1.

• 8

Divide the sum of the squares by N - 1 to determine the variance.

## Determine Precision

• 9

Take the square root of the variance to get the standard deviation.

• 10

Multiply the standard deviation by 100 to convert to percentage.

• 11

If the data values fall within one standard deviation below or above the average then they are considered precise values.

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## Resources

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