Calibration Model Wizard
- Step 5 - Range Selection/Data extraction
Step 5 assists the user in selecting a significant spectral range for
the calibration. Finding the significant spectral ranges is the most challenging
part of the calibration model setup since the chosen area strongly influences
the quality of the resulting model. The software assists the user in finding
the optimal spectral regions to create a good calibration model.
The range selection / data extraction will differ depending on the selected
calibration model. Step 5 of the Calibration Model Wizard will show the
appropriate method for chosen calibration model. The options are:
Data extraction for the univariate model
Spectral range selection for PLS models
Spectral range selection for MLR models
The three different options will in explained in detail in the following
section.
The data extraction for the Univariate Model is described in detail
in the topic Data extraction - Univariate Calibration.
Please refer to this
topic in the chapter Chemometric Analysis.
Spectral range selection for PLS calibration models does not be as accurate
as for MLR the models, because the algorithm will find the optimal spectral
regions by iteration on its own. The user is advised to limit investigations
and modelling to spectral regions of interest in order to avoid misinterpretation.
Thus the user starts with a wide spectral range and will refine selection
during subsequent recalculation of the model.
The initial selection is by default the complete spectral range:
Modifying spectral region
Refinement of the spectral region requires unchecking the "Use
Full Spectral Range" checkbox. A spectral range table is shown with
a default range in the center of the applicable spectral range. User defined
spectral ranges can be adjusted easily either graphically by adjusting
the range selector with the mouse or by directly entering numerical values
in the table.
This example uses the automatically proposed ranges.
MLR is a very sensitive calibration model providing excellent and reliable
results if parameters are properly adjusted. There are several and mostly
time consuming automatic algorithms available for
this purpose, but the most reliable results will be obtained by manual
selection.
The wizard will assist the user during spectral range selection which
is an interactive process strongly related to cross correlations between
highly correlated spectral regions. Initially the software proposes the
optimal and highest correlated spectral range. After fine tuning by the
user the next spectral range is proposed. These steps are repeated until
no more cross correlations are detected.
This procedure will prevent the user from over fitting the calibration
model or selecting non-correlated spectral ranges. Furthermore spectral
regions are very narrow and can be precisely defined according to the
quality of spectral data.
The corresponding dialog looks like this:
Contents
In the data view, the average spectrum (red line), the standard deviation
shape (light red shape) and the correlation spectrum (grey line) are displayed
as described before. Additionally, the initial spectral range proposed
by the software is shown as vertical light blue rectangle shape (at approx.
1550 wave numbers in the picture above).
In the table below details of the first proposed spectral range are
shown.
After proposal of a spectral range by the software, the user must tune
the precise position of the range to fit the peak position of interest
best. For this purpose, the user should zoom
into the region around the selected spectral range:
The center of the selected spectral region is emphasized by a vertical
red line. Start and end markers are indicated by little blue squares.
All items (the spectral range, start marker and end marker) can be moved
to desired positions within the spectrum using drag&drop with the
Left
mouse button.
Alternatively, the precise values can be directly entered into the particular
fields of the spectral range table below the data view.
Automatic proposal of spectral ranges!
The software always proposes a symmetrical
spectral range with an absolute size of five data points. The user may
change the number of data points or select different start and end markers,
so that an asymmetric range is applied. This strongly depends on the peak
requirements.
After proper adjustment of the first spectral range, further ranges
can be added by clicking the New button.
The next spectral range is proposed automatically and the correlation
spectrum is updated accordingly. The slope of correlation spectrum changes,
because already selected spectral ranges are no longer considered in calculation.
The actual correlation spectrum will help the user to identify the next
significant range and it also helps to prevent selection of cross correlated
regions.
The new proposed range is shown as light blue rectangle, whereby the
color of the last one has changed into light green.
Again, the position and the size of the actual spectral region can be
changed by the user.
Changing previously selected spectral ranges!
Previously adjusted ranges affect all subsequent
ranges. The software will display a warning if the user tries to modify
the previously selected ranges. An earlier range may be modified if the
later ranges are discarded.
The user can add as many new spectral ranges as he likes, but adding
too many of them will over fit the model. In this case cross correlations
will reduce the quality of the final result. The user will be warned by
the software automatically, if this limit is reached.
The warning limit depends on the Overfitting Threshold. The parameter
can be adjusted in the corresponding box in the dialog.
After confirming the message by clicking the OK button, the new spectral range is added
anyway to give the user the freedom of choice. It should be deleted again
to ensure integrity of the model.
Deletion of spectral ranges is performed as described in the following:
Select the desired
range either in the data view
or in the spectral ranges table.
Click on the Delete-button or press the Del-key on your keyboardto delete it.
Deletion must only be applied to the last / actual range in the list.
Can I remove intermediate spectral ranges?
If any
intermediate range is removed from the list, the whole selection process
will collapse and any successors must be removed as well in order to avoid
over fitting or misinterpretation.
When using the manual background noise correction the range selection
differs slightly from the automatic workflow. If the Manual Noise Correction
checkbox has been activated, the range selection dialog will look like
this:
Again, the software will propose a spectral region for the calibration
(vertical red line with blue border) and the user may change the position
and extent of this region. Additionally a second region for the background
noise correction (vertical red line with yellow border) will be proposed.
The position and extent of this selection may also be edited by the user.
Ideally, the select region should lie in a preferably non-correlated area
with similar background noise as the calibration region. Regions close
to the selected calibration region should be preferred for noise correction.
Navigation
Just click the Next
> button to proceed to the next step.
Clicking the Cancel button will
abort creating a new calibration.
