Variance
Definition
The average of the squared deviations from the mean. Because it squares the original unit of measurement, variance is not directly interpretable in the original units, but it is the basis for the standard deviation.
- Field
- Descriptive statistics
- Formula basis
- Mean of squared deviations from the mean
- Units
- Squared units of the original measurement
- Related measure
- Standard deviation equals the square root of variance
Common questions
Why do forensic statisticians usually report standard deviation instead of variance?+
Standard deviation is expressed in the same units as the original measurement, such as millimetres or nanograms, so it can be compared directly to the data and communicated to a non-statistical audience. Variance, being in squared units, is a computational stepping stone rather than a figure typically quoted in a report.
What does a large variance indicate about a forensic dataset, such as a set of repeated instrument readings?+
A large variance signals that individual measurements are widely spread around the mean, which can reflect genuine sample heterogeneity, poor measurement precision, or an unstable analytical method. Investigating the cause is important before treating the mean as representative of the data.
Related terms
- Mean
- The arithmetic average of a dataset, calculated as the sum of all values divided by the number of values. Sensitive to extreme...
- 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...
- Standard Deviation
- The square root of the variance. Expressed in the same units as the original measurement, making it interpretable as a typical distance...