# [QUANTITY] Why quantities always have errors

Ed Shaya Edward.J.Shaya.1 at gsfc.nasa.gov
Tue Nov 18 10:50:02 PST 2003

```Need to add to my wish list:

Errors can be given as a fraction or percentage of the value
An Error can be for a value or for all the values.
Errors may be generated by algorithm, as in sqrt(N).

Ed

Ed Shaya wrote:

>
>  what things are going to be in the Error before we can know what sort
> of beast it is.  We should create a wish list of things that COULD go
> into an Error object.  Then we can go over it and strike things that
> are either too rare to bother with or just too complicated.  Here is
> my wish list:
>
> positiveError:  error bar to the plus side
> negativeError:  error bar to the minus side
> Error:  for symmetric error bars
> sigma:  number of standard deviations that the  Error represents
> confidence:  percentage confidence level that the Error represents
> distributionFunction:  gaussian, laurentzian, hat-function, etc
>
> PgtV: Sometimes error is presented as the probability of exceeding
> the given value
> PltV:  or not reaching the given value
>
> And, one may have the entire probability function as the quantity in
> the Error.
>
> All of the above for each of
> randomError: Error resulting from finite precision of the measurement
> systematicError:  inaccuracy intoduced by faulty equipment,
> calibration, or technique.
> significantFigures: number of significant digits in the number
>
> We also need to deal with the issue of an upperLimit or lowerLimit,
> [which, by the way, usually does not have an error because it is
> either the error or a measured value plus an error.  But, it can have
> an error which is actually the error-on-the-error.]
>
> Ed
>

```