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# NCL: Probability Distribution Functions

The probability distribution (frequency of occurrence) of an individual
variable,

**X**, may be obtained via the

**pdfx** function.
Given two variables

**X** and

**Y**, the bivariate
joint probability distribution returned by the

**pdfxy**
function indicates the probability of occurrence defined in terms of both

**X** and

**Y**.

Generally, the larger the array(s) the smoother the derived PDF.
Bin sizes of less-than [greater-than] the default number of 25 bins will
result in smoother [rougher] plots.

pdf_1.ncl:
Using the

**pdfx** function,
this example illustrates univariate PDFs from three
variables with three different distributions.
Default settings of parameters are used (

*eg.*, 25 bins).

pdf_2.ncl:
This illustrates using a user specified number of bins.
Here, 40 bins are specified. This results in a more ragged
view of the distribution.
Use of the returned

*bin_center*
attributes from three PDFs to place all on
a common x-axis is illustrated. (Minor changes would be required if
the number of bins used had been different.)
The

*gsnXYBarChart*
and

*gsnXYBarChartOutlineOnly*
illustrate using a bar style plot.

pdf_3.ncl:
Using the

**pdfxy** function,
illustrate a simple bivariate PDF using two variables having normal
distributions.

pdf_4.ncl:
Similar to Example 3 but use different bin numbers.
Given a fixed number of values, the fewer bins used, the smoother
the resulting PDF.

pdf_5.ncl:
The bivariate distributions of variables from variables with different
univariate distributions will yield different patterns.
Here, the univariate distributions of Example 1 are used
to create bivariate PDFs.

Some tuning of plots may be necessary to focus on regions of interest.
Here, the "Gamma/Chi" distributions are highly skewed. There are
large areas where the joint probabilites are near or at zero.
NCL coordinate subscripting is used to select regions of interest.

pdf_6.ncl:
Variables that may not be continuous [probabilities=0.0]
may be best viewed via use of "raster" plots. These clearly show
the bin and data resolution.

Note that using **gsn_csm_contour** results in
the raster bins at the edges being reduced to half width. The use of
**plt_pdfxy** located in the **shea_util** expands the
contour area and allows the edge raster bins to be fully viewed.