varIdent(nlme)
varIdent()所属R语言包：nlme
Constant Variance Function
恒定方差函数
译者：生物统计家园网 机器人LoveR
描述----------Description----------
This function is a constructor for the varIdent class, representing a constant variance function structure. If no grouping factor is present in form, the variance function is constant and equal to one, and no coefficients required to represent it. When form includes a grouping factor with M > 1 levels, the variance function allows M different variances, one for each level of the factor. For identifiability reasons, the coefficients of the variance function represent the ratios between the variances and a reference variance (corresponding to a reference group level). Therefore, only M-1 coefficients are needed to represent the variance function. By default, if the elements in value are unnamed, the first group level is taken as the reference level.
此功能一个varIdent类代表一个常数方差的功能结构，构造。如果没有分组因素是目前在form，方差函数是常数，等于一体，并要求代表它没有系数。当form包括M > 1水平的分组因素，方差函数允许M个不同的差异，为每个水平的因素之一。为辨识的原因，方差函数的系数代表之间的差异和参考方差（对应参考组级别）的比率。因此，只有M-1系数代表方差函数。默认情况下，如果在元素value是无名的，第一组的水平作为参考水平。
用法----------Usage----------
varIdent(value, form, fixed)
参数----------Arguments----------
参数：value
an optional numeric vector, or list of numeric values, with the variance function coefficients. If no grouping factor is present in form, this argument is ignored, as the resulting variance function contains no coefficients. If value has length one, its value is repeated for all coefficients in the variance function. If value has length greater than one, it must have length equal to the number of grouping levels minus one and names which identify its elements to the levels of the grouping factor. Only positive values are allowed for this argument. Default is numeric(0), which results in a vector of zeros of appropriate length being assigned to the coefficients when object is initialized (corresponding to constant variance equal to one).
一个可选的数字向量，或数值的列表，与方差函数系数。如果没有分组因素是form，则忽略此参数，方差函数不包含系数。如果value有一个长度，其值重复方差函数系数。如果value有长度大于一，它必须有长度等于分组级别减一和名称确定其元素分组因素水平。这种说法只允许正值。默认是的numeric(0)，这导致一个系数时object初始化被分配到适当的长度为零的向量（对应常数方差等于一）。
参数：form
an optional one-sided formula of the form ~ v, or ~ v | g, specifying a variance covariate v and, optionally, a grouping factor g for the coefficients. The variance covariate is ignored in this variance function. When a grouping factor is present in form, a different coefficient value is used for each of its levels less one reference level (see description section below). Several grouping variables may be simultaneously specified, separated by the * operator, like in ~ v | g1 * g2 * g3. In this case, the levels of each grouping variable are pasted together and the resulting factor is used to group the observations. Defaults to ~ 1.
可选片面公式的形式~ v或~ v | g，指定方差协v“，可以选择，分组因素g系数。在这个方差函数的方差协变量被忽略。当一个分组的因素是目前在form，不同系数的值用于每个水平少了一个参考水平（见下文说明）。可以同时指定几个分组变量，分离，像*~ v | g1 * g2 * g3运营商。在这种情况下，每个分组变量的水平都粘在一起，由此产生的因素是用来组的意见。 ~ 1默认。
参数：fixed
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. It must have names identifying which coefficients are to be fixed. Coefficients included in fixed are not allowed to vary during the optimization of an objective function. Defaults to NULL, corresponding to no fixed coefficients.
一个可选的数字向量，或数值，指定其中的部分或全部方差函数的系数应该是固定的值列表。它必须有确定哪些系数是固定的名称。系数包括fixed不允许不同的目标函数的优化过程中的。默认NULL，没有固定的系数。
值----------Value----------
a varIdent object representing a constant variance function structure, also inheriting from class varFunc.
一个varIdent对象代表一个常数方差函数的结构，也继承类varFunc。
作者（S）----------Author(s)----------
Jose Pinheiro and Douglas Bates <a href="mailto:bates@stat.wisc.edu">bates@stat.wisc.edu</a>
参考文献----------References----------
in S and S-PLUS", Springer.
参见----------See Also----------
varClasses, varWeights.varFunc,
varClasses，varWeights.varFunc
举例----------Examples----------
vf1 <- varIdent(c(Female = 0.5), form = ~ 1 | Sex)
转载请注明:出自 生物统计家园网(http://www.biostatistic.net)。
注：
注1：为了方便大家学习，本文档为生物统计家园网机器人LoveR翻译而成，仅供个人R语言学习参考使用，生物统计家园保留版权。
注2：由于是机器人自动翻译，难免有不准确之处，使用时仔细对照中、英文内容进行反复理解，可以帮助R语言的学习。
注3：如遇到不准确之处，请在本贴的后面进行回帖，我们会逐渐进行修订。
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发表于 2012-2-17 10:04:29
