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Cumulative Distribution Function
CDF, or
Cumulative
Distribution
Function plots display exactly the same information
as do histograms. The difference is that the histogram values are
summed as the fluorescence intensity increases; thus, the CDF begins
at 0% (left axis) and ends at 100% (right axis). You can change the
vertical scaling (to expand a portion of the CDF plot) by setting
the appropriate fields in the
Graph
Specification Dialog
(click
i on the graph window to show this dialog).
CDF plots are extremely useful for quickly finding the fluorescence
of a distribution corresponding to any given percentile (such as the
median). The sample plot below shows how to find the median (50th
percentile), 75th, and 90th percentiles for this data. Note that
unlike the histogram, the Y-axis is labeled in units of relative
frequency. In this example, the median fluorescence of the
population is about 6.0 fluorescence units.
The plot below overlays the CDF and the histogram for the
same data. (This overlay was done by copying the graphs from FlowJo
into Canvas and overlaying them in that program).
See also the page on histograms.
