BA.est {MethComp}R Documentation

Bias and variance components for a Bland-Altman plot.

Description

A variance component model is fitted to method comparison data with replicate measurements in each method by item stratum. The purpose is to simplify the construction of a correct Bland-Altman-plot.

Usage

  BA.est( data, linked=TRUE, exch=!linked, MI=TRUE )
  

Arguments

data A data frame representing method comparison data with replicate measurements, i.e. with variables meth, item, repl and y.
linked Are the replicated linked within item across methods?
exch Are the replicated exchangeable within (item,methods)? This argument is just for convenience.
MI Logical. Should the method by item interaction be included in the model.

Details

The model fitted is:

y=alpha_m + mu_i + c_mi + a_ir + e_ir, var(c_mi)=tau_m^2, var(a_ir)=nu^2, var(e_mir)=sigma_m^2

We can only fit separate variances for the tau's if more than two methods are compared (i.e. nM > 2).

NOTE: Currently, the author is too ignorant about the ins and outs of lme to find out how to get different tau's when nM>2. Therefore the estimates in this case are equal i.e. from the model postulated.

Value

A list with four elements:

Bias Vector of estimates of alpha_m, the first element is always 0.
Var.comp Two-column matrix of sds and variances of the variance components, nM tau's, one nu if linked, and nM sigma's. Only those in the model specified are included.
LoA Four-column matrix with mean difference, lover and upper limit of agreement and prediction SD. Each row in the matrix represents a pair of methods.
Rep.coef Two-column matrix of repeatability SDs and repeatability coefficients (2.77*SD).

Author(s)

Bendix Carstensen

References

Carstensen, Simpson & Gurrin: Statistical models for assessing agreement in method comparison studies with replicate measurements, Preprent, Departmet of Biostatistics, University of Copenhagen, 2007, http://www.pubhealth.ku.dk/bs/publikationer/Research_report_07-14.pdf/.

See Also

BA.plot,tab.repl,perm.repl

Examples

data( ox )
BA.est( ox )
BA.est( ox, exch=TRUE )
data( sbp )
BA.est( sbp )
BA.est( sbp, exch=TRUE )

[Package MethComp version 0.3.0 Index]