startEmax {clinDR}

Compute starting parameter values for the 3- or 4- Emax model.

Description

Compute starting parameter values for iterative procedures for estimating parameters of the 3- or 4- parameter Emax model

Usage

startEmax(y,
          dose,
          baseline,
          count=rep(1,length(y)),
          modType=3,
          binary=FALSE,
          lbED50=doselev[2]/10,
          ubED50=max(doselev),
          lbLambda=0.5,
          ubLambda=5)

Arguments

y

Outcome (response) variable for the Emax modeling.

binary

The default is continuous (binary=FALSE). When (binary=TRUE), y must be 0/1 and starting values are returned for an Emax model on the logit scale.

dose

Dose variable corresponding to each outcome value.

baseline

Optional baseline covariate(s) of same length as y. When baseline is specified, starting values are created from anacova adjusted dose group means.

count

Counts for the number of patients with each dose/y value. Default is 1 (ungrouped data).

modType

modType=3 (default) for the 3-parameter Emax model. modType=4 for the 4-parameter Emax model.

lbED50

If the starting ED50 is below lbED50, it is set to lbED50.

ubED50

If the starting ED50 is above ubED50, it is set to ubED50.

lbLambda

If the starting lambda is below lbLambda, it is set to lbLambda.

ubLambda

If the starting lambda is above ubLambda, it is set to ubLambda.

Value

Returns a vector with named elements for the starting values for a 3 or 4 parameter Emax model. The order is log(ED50), (lambda, 4 parm), emax, and e0. If baseline is specified, a 'beta' starting parameter is also returned at the end of the vector.

Note

The method is modified from functions created by J. Rogers and start functions supplied with R (SSfp1). The ED50 (and lambda) are computed using the logit-linear relationship between the proportion of the mean response out of the max response and the log(dose). The method assumes placebo data are present, but it will return a starting value even if it is not present. A miniumum of four dose levels is required for 4-parameter starting values.

Author(s)

Neal Thomas

See Also

nls, emaxalt

Examples

data("metaData")
exdat<-metaData[metaData$taid==6 & metaData$poptype==1,]

startEmax(exdat$rslt,exdat$dose)