proc pvalue


Version 2.41 Mar2009

Scripts


Manual page for proc_pvalue(PL)
proc pvalue computes and displays p values associated with a statistical t test (New in version 2.41). A t test compares two groups of numeric scores; the associated p value is the probability that there is no significant difference between group1 and group2. A low p value (typically < 0.05 or < 0.01) indicates that there's a sufficiently significant difference between group1 and group2. This is useful for example, in treatment vs. control studies in being able to say that the treatment made a difference or not.

How your data must be organized: Each data row must have fields for mean, SD, and N, for group1 and group2, plus a X category identifier field (total of 7 fields required). The mean, SD and N can be computed from raw data using proc processdata (action=summary), and then joined (using proc processdata again, action=join) so that data for both groups are together on each row. See the above example which uses this technique.

This proc will compute one p value per data row, and display it at the X category location matching the group identifier field (so X scaletype must be categories). By default the p value will be displayed using the format eg. "p=0.12", along the bottom of the plot area, and statistically significant p values are highlighted in yellow.




Attributes

xfield, statfields1, and statfields2 must be specified.

xfield     dfield

Data field containing an identifier for determining where in X the p value will be displayed.


statfields1     meanfield SDfield Nfield

Three data fields containing mean, SD, and N for group1.


statfields2     meanfield SDfield Ndfield

Three data fields containing mean, SD, and N for group2.


textdetails     textdetails

Size, color, etc. of the p value string. This attribute can also be used to adjust the location of the displayed string.
Example: textdetails: size=7 adjust=0,-0.25


printformat     string

Controls formatting of the result string and the pvalue numeric. Default is p=%2.2f.
Example: printformat: p = %3.3f


lowp     h

Mathematically, p values can never reach absolute 0. Convention holds that p values less than a certain threshold be displayed as eg. p < 0.01 This attribute controls that threshold. Default is 0.01
Example: lowp: 0.005


signif     h

Convention holds that p values less than a certain threshold indicate a sufficiently significant difference between the two groups. By default this proc highlights these in yellow. Default is 0.05.
Example: signif: 0.01


signifcolor     color | none

The color to use when highlighting "significant" p values. Default is yellow. To turn off the highlighting feature specify none.


select     select expression

Allows data rows to be selected for inclusion using a selection expression.
Example: @@2 = B





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Markup created by unroff 1.0,    March 11, 2009.