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Edit File: varPower.html
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><title>R: Power Variance Function</title> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <link rel="stylesheet" type="text/css" href="R.css" /> </head><body> <table width="100%" summary="page for varPower {nlme}"><tr><td>varPower {nlme}</td><td style="text-align: right;">R Documentation</td></tr></table> <h2>Power Variance Function</h2> <h3>Description</h3> <p>This function is a constructor for the <code>varPower</code> class, representing a power variance function structure. Letting <i>v</i> denote the variance covariate and <i>s2(v)</i> denote the variance function evaluated at <i>v</i>, the power variance function is defined as <i>s2(v) = |v|^(2*t)</i>, where <i>t</i> is the variance function coefficient. When a grouping factor is present, a different <i>t</i> is used for each factor level. </p> <h3>Usage</h3> <pre> varPower(value, form, fixed) </pre> <h3>Arguments</h3> <table summary="R argblock"> <tr valign="top"><td><code>value</code></td> <td> <p>an optional numeric vector, or list of numeric values, with the variance function coefficients. <code>Value</code> must have length one, unless a grouping factor is specified in <code>form</code>. If <code>value</code> has length greater than one, it must have names which identify its elements to the levels of the grouping factor defined in <code>form</code>. If a grouping factor is present in <code>form</code> and <code>value</code> has length one, its value will be assigned to all grouping levels. Default is <code>numeric(0)</code>, which results in a vector of zeros of appropriate length being assigned to the coefficients when <code>object</code> is initialized (corresponding to constant variance equal to one).</p> </td></tr> <tr valign="top"><td><code>form</code></td> <td> <p>an optional one-sided formula of the form <code>~ v</code>, or <code>~ v | g</code>, specifying a variance covariate <code>v</code> and, optionally, a grouping factor <code>g</code> for the coefficients. The variance covariate must evaluate to a numeric vector and may involve expressions using <code>"."</code>, representing a fitted model object from which fitted values (<code>fitted(.)</code>) and residuals (<code>resid(.)</code>) can be extracted (this allows the variance covariate to be updated during the optimization of an object function). When a grouping factor is present in <code>form</code>, a different coefficient value is used for each of its levels. Several grouping variables may be simultaneously specified, separated by the <code>*</code> operator, like in <code>~ v | g1 * g2 * g3</code>. In this case, the levels of each grouping variable are pasted together and the resulting factor is used to group the observations. Defaults to <code>~ fitted(.)</code> representing a variance covariate given by the fitted values of a fitted model object and no grouping factor. </p> </td></tr> <tr valign="top"><td><code>fixed</code></td> <td> <p>an optional numeric vector, or list of numeric values, specifying the values at which some or all of the coefficients in the variance function should be fixed. If a grouping factor is specified in <code>form</code>, <code>fixed</code> must have names identifying which coefficients are to be fixed. Coefficients included in <code>fixed</code> are not allowed to vary during the optimization of an objective function. Defaults to <code>NULL</code>, corresponding to no fixed coefficients.</p> </td></tr> </table> <h3>Value</h3> <p>a <code>varPower</code> object representing a power variance function structure, also inheriting from class <code>varFunc</code>. </p> <h3>Author(s)</h3> <p>José Pinheiro and Douglas Bates <a href="mailto:bates@stat.wisc.edu">bates@stat.wisc.edu</a></p> <h3>References</h3> <p>Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models in S and S-PLUS", Springer. </p> <h3>See Also</h3> <p><code><a href="varWeights.html">varWeights.varFunc</a></code>, <code><a href="coef.varFunc.html">coef.varPower</a></code></p> <h3>Examples</h3> <pre> vf1 <- varPower(0.2, form = ~age|Sex) </pre> <hr /><div style="text-align: center;">[Package <em>nlme</em> version 3.1-139 <a href="00Index.html">Index</a>]</div> </body></html>