DA2y: Convert DA to (classical) regression
In MethComp: Analysis of Agreement in Method Comparison Studies

Description Usage Arguments Details Value Author(s) References Examples

The functions DA2y and y2DA are convenience functions that convert the estimates of intercept, slope and sd from the regression of D=y1-y2 on A=(y1+y2)/2, back and forth to the resulting intercept, slope and sd in the relationship between y1 and y2, cf. Carstensen (2010), equation 6.

1	DA2y(a = 0, b = 0, s = NA)

`a`	Intercept in the linear relation of the differences `y1-y2` to the averages `(y1+y2)/2`. If a vector of length>1, this is used instead of `a`, `b` and `s`, and `b` and `s` are ignored.
`b`	Slope in the linear relstion of the differences to the averages.
`s`	SD from the regression of the differences in the averages. Can be `NA`.

DA2y takes the intercept(a), slope(b) and sd(s) from the relationship (y1-y2)=a+b((y1+y2)/2)+e with sd(e)=s, and returns a two by 3 matrix with columns "int","slope","sd" and rows "y1|2","y2|1".

DA2y returns a 2 by 3 matrix with rownames c("y1|2","y2|1") and columnnames c("int","slope","sd"), calculated under the assumption that the differences were formed as D <- y1 - y2.

Bendix Carstensen, Steno Diabetes Center, bendix.carstensen@regionh.dk, http://BendixCarstensen.com/MethComp

B. Carstensen: Comparing methods of measurement: Extending the LoA by regression. Stat Med, 29:401-410, 2010.

data( milk )
DA.reg( milk )
data( sbp )
print( DA.reg(sbp), digits=3 )
# Slope, intercept : y1 = 0.7 + 1.2*y2 (0.4)
A <- c(0.7,1.2,0.4)
( y2DA( A ) )
( DA2y( y2DA( A ) ) )