Fixing baselines

  •    CORRECT BASELINE...   B          (Active channel only)   There are seven methods of baseline correction.  You should select the one most appropriate for your data set.   In all, the goal is to set the baseline (reference) values to zero.

    1.  Automatic -- The program selects blocks at the start and/or end of the data and uses them in a regression to compute the baseline.  You can change the number of cases in these blocks by editing the value in the box (the default is 15).  Use the automatic method only if the beginning and end of the file contain nothing but true baseline values.  If this condition is not met, use another method (such as #2 or #3, below).

    2.  Automatic (periodic) -- (shown at right)    The program uses one of two methods to identify baselines:

    (a)   The program uses markers in each reference as a cue for baseline setting.  You can chose which marker is the reference indicator, and whether the reference adjustment should begin on either side of that marker (and by how much).  In my experience, this usually works more reliably than the second method and can deal with user-induced alterations in the reference vs. sampling schedule, but it does require markers at the appropriate points in the file.   The LabHelper and Sable Systems acquisition programs will do that automatically, if properly set up.

    (b)   You use the cursor to select a starting point and time interval between reference readings, and the program uses them to 'step' from reference to reference.   This method works well if all reference intervals in the file are exactly the same and it does not require any markers to indicate references.

    Only the first method (using markers) can be scripted.

    3.  Use the mouse to select a sequential series of baseline blocks (multiple points), starting from the 'left' end of the file and moving to the 'right' end.  This method corrects the baseline in a series of segments (up to 300 in a given file; you can repeat the process if you need to handle more than that).  Alternately, you can click on single points (instead of selecting blocks).
            NOTE: you must select at least three blocks or points.   For a one- or two-point correction, use the next options.

    4.  Use the mouse to select beginning and ending blocks, which LabAnalyst then uses in a linear regression to compute the baseline.  Alternately, you can click the single point button to pick baseline points (instead of selecting blocks).

    5.  Use the mouse to select a single block which the program uses to calculate a flat baseline.  Alternately, you can click on a single point (instead of selecting a block).

    6.  Keyboard entry of beginning and ending baseline values.

    7.  Keyboard entry of a constant baseline value.

    For all methods except #2 (automatic periodic references) or #3 (multiple blocks), the baseline is drawn on-screen (if it fits within the current Y-axis scale) and you can cancel, reselect, or accept it.  The final baseline is drawn across the entire file.

    See the Reference removal section for an easy method of eliminating baseline sections for analysis.

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