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help for sslope
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Simple Slope Calculation for Linear Regression

sslope varlist [if exp] [in range] , [i(x z xz)] [i(x z w xz xw zw
xzw)] [i(x x-squared)] [i(x z x-squared xz)] [i(z x x-squared xz)]
[sd(#)] [Graph] or [go(graph options)] [Fits]

varlist = regression equation with centered variables including all
interactions

option i must include either (x z xz) (2-way interaction), gives slope
of criterion variable on x at levels of z (x z xz are variable names)

OR

(x z w xz xw zw xzw) (3-way interaction), gives slope of criterion
variable on x at levels of z and w

OR

(x x-squared) (quadratic), gives slope of criterion variable on x at
levels of x. Also calculates the minimum or maximum of the curve.

OR

(x z x-squared xz) (quadratic plus linear interaction), gives slope of
criterion variable on x at levels of x and z. Also calculates the
minumum or maximum of the curves.

OR

(z x x-squared xz) (quadratic plus linear interaction), gives slope of
criterion variable on z at levels of x.

Description

This calculates simple slopes for interactions between continuous variables in
linear regression analysis.  For example, the slope of the criterion on x
conditional upon z. All continuous variables in the regression must be centered
at the mean. The simple slope of interest must be specified in i. This must be
of the form x(slope of interest) z(moderating variable) xz(the interaction
between the two). Hence, i(x z xz) provides a different result than i(z x xz).
Both are correct, one is the slope of y on x conditional upon z and the other,
the slope of y on z conditional upon x. However, i(x xz z) will produce
incorrect results.  Similarly, for a three-way interaction, i must be in the
form of (x z w xz xw zw xzw) for the slope of x conditional upon z and w.
Important, the order i(x z w xz wz xw xzw) will not produce the correct
results. The order is x (slope of interest) z w (first order terms of the
moderators) x^2 (x quadratic term) xz xw (2-way interactions with x) zw (the
2-way interaction of the moderators without x) xzw (the three way interaction).
The order of variables in varlist does not matter but calculations of the
simple slopes depend upon the specified order in i. Simple slopes associated
with both an interaction and a quadratic term (i.e., y on x when there are both
xz and x-squared terms) are not supported at this time.  The calculations are
based upon the formulas provided in Aiken & West (1991).

Options

sd(#) specifies the conditional levels of interest of z and/or w, in sd units,
default = 1.0.

Graph requests a graph of the conditional slopes. Continuous variables not
included in the interaction are held constant at mean=0, if there are
dichotomous variables in the model, this option creates graphs for a
category=0, if there is not a category=0, the intercepts will be invalid.

go requests a graph as above, but accepts all standard graph options for
scatter.  Choose either Graph for the default graph OR go and define graph
options.

Fits requests the conditional fitted values be saved. This is included to

Examples

sslope y x z xz, i(x z xz)
sslope y x z xz, i(x z xz) sd(2)
sslope y x z w xz xw zw xzw, i(x z w xz xw zw xzw)
sslope y q x z w xz, i(x z xz)
sslope y x x-squared, i(x x-squared) graph
sslope y x z x-squared, i(x x-squared) graph fits
sslope y x q z  xz x-squared, i(x z x-squared xz) go(sort c(. l l l) m(o i i i)
>  mlabel(id)) fits

sslope dialog box

Author

Jeffrey S. Simons - jsimons@usd.edu

Also see

mcenter
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