gamlss (version 4.2-4)

centiles.pred: Creating predictive centiles values

Description

This function creates predictive centiles curves for new x-values given a GAMLSS fitted model. The function has three options: i) for given new x-values and given percentage centiles calculates a matrix containing the centiles values for y, ii) for given new x-values and standard normalized centile values calculates a matrix containing the centiles values for y, iii) for given new x-values and new y-values calculates the z-scores. A restriction of the function is that it applies to models with only one explanatory variable.

Usage

centiles.pred(obj, type = c("centiles", "z-scores", "standard-centiles"), 
             xname = NULL, xvalues = NULL, power = NULL, yval = NULL, 
             cent = c(0.4, 2, 10, 25, 50, 75, 90, 98, 99.6), 
             dev = c(-4, -3, -2, -1, 0, 1, 2, 3, 4), 
             plot = FALSE, legend = TRUE, 
             ...)

Arguments

obj

a fitted gamlss object from fitting a gamlss continuous distribution

type

the default, "centiles", gets the centiles values given in the option cent. type="standard-centiles" gets the standard centiles given in the dev. type="z-scores" gets the z-s

xname

the name of the unique explanatory variable (it has to be the same as in the original fitted model)

xvalues

the new values for the explanatory variable where the prediction will take place

power

if power transformation is needed (but read the note below)

yval

the response values for a given x required for the calculation of "z-scores"

cent

a vector with elements the % centile values for which the centile curves have to be evaluated

dev

a vector with elements the standard normalized values for which the centile curves have to be evaluated in the option type="standard-centiles"

plot

whether to plot the "centiles" or the "standard-centiles", the default is plot=FALSE

legend

whether a legend is required in the plot or not, the default is legent=TRUE

...

for extra arguments

Value

• a vector (for option type="z-scores") or a matrix for options type="centiles" or type="standard-centiles" containing the appropriate values

Warning

See example below of how to use the function when power transofrmation is used for the x-variables

References

Rigby, R. A. and Stasinopoulos D. M. (2005). Generalized additive models for location, scale and shape,(with discussion), Appl. Statist., 54, part 3, pp 507-554.

Stasinopoulos D. M., Rigby R.A. and Akantziliotou C. (2006) Instructions on how to use the GAMLSS package in R. Accompanying documentation in the current GAMLSS help files, (see also http://www.gamlss.org/).

Stasinopoulos D. M. Rigby R.A. (2007) Generalized additive models for location scale and shape (GAMLSS) in R. Journal of Statistical Software, Vol. 23, Issue 7, Dec 2007, http://www.jstatsoft.org/v23/i07.

See Also

gamlss, centiles, centiles.split

Examples

# bring the data and fit the model
data(abdom)
a<-gamlss(y~pb(x),sigma.fo=~pb(x), data=abdom, family=BCT)
#plot the centiles
centiles(a,xvar=abdom$x)
# calculate the centiles at new x values 
newx<-seq(12,40,2)
mat <- centiles.pred(a, xname="x", xvalues=newx )
mat
# now plot the centiles  
 mat <- centiles.pred(a, xname="x",xvalues=newx, plot=TRUE )
# calculate standard-centiles for new x values using the fitted model
newx <- seq(12,40,2)
 mat <- centiles.pred(a, xname="x",xvalues=newx, type="standard-centiles" )
 mat
# now plot the centiles  
mat <- centiles.pred(a, xname="x",xvalues=newx, type="s", plot = TRUE )
# create new y and x values and plot them in the previous plot
newx <- c(20,21.2,23,20.9,24.2,24.1,25)
newy <- c(130,121,123,125,140,145,150)
for(i in 1:7) points(newx[i],newy[i],col="blue")

# now calculate their z-scores
znewx <- centiles.pred(a, xname="x",xvalues=newx,yval=newy, type="z-scores" )
znewx
# now with transformed x-variable within the formula
aa<-gamlss(y~pb(x^0.5),sigma.fo=~pb(x^0.5), data=abdom, family=BCT)  
centiles(aa,xvar=abdom$x)
mat <- centiles.pred(aa, xname="x",xvalues=c(30) )
xx<-rep(mat[,1],9)
yy<-mat[,2:10]
points(xx,yy,col="red")
# now with x-variable previously transformed 
nx<-abdom$x^0.5
aa<-gamlss(y~pb(nx),sigma.fo=~pb(nx), data=abdom, family=BCT)
centiles(aa, xvar=abdom$x)
newd<-data.frame( abdom, nx=abdom$x^0.5)
mat <-  centiles.pred(aa, xname="nx", xvalues=c(30), power=0.5, data=newd)
xxx<-rep(mat[,1],9)
yyy<-mat[,2:10]
points(xxx,yyy,col="red")