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How to Calculate Reference Intervals

A reference range gives you an idea of what a normal measurement is for a particular data set. For example, knowing the reference range can help a school district determine appropriate placement in special education classes, or it could help a doctor decide whether lab test results are normal or abnormal. A reference range is calculated using the standard deviation (a measure of how spread out the data is) and the data’s mean (often called an “average”).

Existing data, including standard deviation and mean

Instructions

- 1
  Multiply the data set's standard deviation by 2. For example, if the standard deviation is 0.67, then 0.67 x 2 = 1.34.
- 2
  Subtract your answer in Step 1 from the data's mean. This is the lower value of the reference interval. For example, if your mean is 4.06, then 4.06 - 1.34 = 2.72.
- 3
  Add your answer from Step 1 to the data's mean. This is the upper value of the reference interval. For example, if your mean is 4.06, then 4.06 + 1.34 = 5.4. Since the reference interval is an interval centered on the mean, plus or minus 2 sigma, we're done; our interval is {2.72, 5.4}.

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