## Error Scan Mode

xyscan provides not only an easy way to scan data points but also error bars. You can either treat the errors as asymmetric or symmetric depending on the values selected in the Error Scan Mode tab in the Tool Box (see below). However, even in the symmetric case, upper and lower (left and right) error bars are always scanned separately. xyscan provides different ways to derive symmetric errors from asymmetric ones.

The error scan modes are:

`No Scan`
  No error bar scan will be performed.
`Asymmetric`
  Lower and upper (left and right) error bars are scanned and stored separately.
`Symmetric(mean)`
  Lower and upper (left and right) error bars are scanned separately but only the average (arithmetic mean) of both is recorded. Choose this option if you are sure the errors are symmetric and you want to increase the precision on the obtained values.
`Symmetric (max)`
  Lower and upper (left and right) error bars are scanned but only the largest of the two is stored. Choose this option when you want to play safe and especially if the image/plot is of pour quality.

You can also scan more than one x(y)-error. It is quite common that scientific plots depict two or more error bars per point (typically in y) to indicate statistical and systematic uncertainties. To scan more than one, press the "+" button next to pull-down menu with which you defined the mode of the first error bar. A second pull-down menu will be displayed that lets you define the mode for the next error bar scan. Use the "-" button to remove the added mode-selection menu. You an add as many error bars as you like, although more than two are relatively rare.

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