Standard Deviation
Definition
The square root of the variance. Expressed in the same units as the original measurement, making it interpretable as a typical distance from the mean. The most commonly reported measure of spread in forensic data.
- Formula
- Square root of the variance
- Units
- Same as the original measurement
- Interpretation
- Typical distance of a value from the mean
- Usage
- Most commonly reported spread measure in forensic data
Common questions
Why is standard deviation preferred over variance for reporting forensic data?+
Variance is expressed in squared units, which are not intuitive to interpret. Standard deviation returns to the original unit of measurement, so a reader can directly compare it to the mean and judge how tightly the data cluster.
Does a small standard deviation always mean measurements are accurate?+
No. It only shows the measurements are consistent with each other, which is precision. A small standard deviation around a value that is systematically off due to a calibration error still reflects poor accuracy despite high precision.
Related terms
- Mean
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- Median
- The middle value of a dataset sorted in ascending or descending order. For an even number of values, it is the average...
- Mode
- The value that appears most frequently in a dataset. Most informative for discrete or categorical data; for continuous measurement data it is...
- Refractive Index (RI)
- A physical property of glass measuring how much light bends as it passes through. RI is the primary discriminating characteristic used in...
- Variance
- The average of the squared deviations from the mean. Because it squares the original unit of measurement, variance is not directly interpretable...