Re: [Phys-l] each error bar == one standard deviation

["Folkerts, Timothy J" <FolkertsT@bartonccc.edu>]

> Unless otherwise specified very explicitly:
> -- For a Gaussian, each error bar is one standard deviation.
>  Note that this is approximately equal to the HWHM.
>  The + and - error bars together cover 68% of the probability.
> -- For a triangular distribution, each error bar is exactly
>   one HWHM.  The + and - error bars together cover 75% of the 
>   probability.
> -- For a square distribution, each error bar is exactly one
>   HWHM.  The + and - error bars together cover 100% of the 
>   probability.

There are other common conventions.  For example, NIST sticks with +/- 1 standard deviation for all of these.  (http://physics.nist.gov/cuu/Uncertainty/typeb.html)
 
"The following figure schematically illustrates the three distributions described above: normal, rectangular, and triangular. In the figures, µt is the expectation or mean of the distribution, and the shaded areas represent ± one standard uncertainty u about the mean. For a normal distribution, ± u encompases about 68 % of the distribution; for a uniform distribution, ± u encompasses about 58 % of the distribution; and for a triangular distribution, ± u encompasses about 65 % of the distribution."


I'm not saying one convent is better than another -- I'm just saying varying conventions are out there.  

Tim Folkerts