Asymmetrical Error Bars

The Starlink NDF standard does not at present cater for asymmetrical errors on either data or axis values. In the absence of such a standard, the ERROR structure, and UWIDTH and LWIDTH components within the AXIS structure have defined for use within ASTERIX. At this stage these are intended solely to allow display of data with asymmetrical error bars, and are supported only by the ASTERIX graphics software (for 1D data only) and by IMPORT, which can be used to generate an NDF with asymmetrical errors from a text file. The additional components will not be recognised by ASTERIX analysis software, and since they fall outside the SGP38 standard, they will not be propagated by Starlink applications (including ASTERIX ones). However, since they are entirely additional components (not changes to existing ones) they will not interfere with the operation of other programs.

An example of a 1D dataset containing asymmetrical errors for both data and axis dimensions is:

        DATASET
          DATA_ARRAY(m)
          ERROR
            UPPER(m)
            LOWER(m)
          AXIS(1)
            DATA_ARRAY(m)
            LWIDTH(m)
            UWIDTH(m)
          LABEL
          UNITS
          etc.

Note that,

• The graphics software will use ERROR in preference to VARIANCE if both are present. (Other software will see only the latter.)

• Similarly LWIDTH/UWIDTH will be used in preference to the standard WIDTH component by graphics only.

• Asymmetrical errors may be specified for either AXIS, or DATA, or both.