A probability distribution that is flat between a lower and an upper limit and zero outside, so every value inside the bounds is treated as equally likely. It is the default for a manufacturer's spec, a resolution limit or a handbook tolerance where nothing suggests values cluster near the center. For a half-width a, the standard uncertainty is a divided by the square root of 3, about 0.577a.
Spec is plus or minus 0.5 degrees with nothing else to go on, so call it a rectangular distribution, 0.29 standard.
When torn between rectangular and triangular, rectangular is the conservative pick and no assessor will fault it.
Also heard as
- uniform distribution
- rectangular
Do not mix it up with
- Triangular distribution: A triangular distribution assumes values bunch toward the center and divides the half-width by the square root of 6 instead of 3.
Related terms
Probability density function, the curve describing how likely each possible value of a quantity is.
Root three
Shop shorthand for dividing a rectangular limit by the square root of 3 to turn it into a standard uncertainty.
Triangular distribution
A distribution peaked at the center and falling linearly to zero at the limits, with a divisor of root six.
Type B evaluation
Sizing an uncertainty component from existing knowledge such as certificates, specs or resolution, with no new repeat data.
Resolution
The finest step an instrument can display, such as one count of the last digit on a digital readout.