[R] Limited number of replicates in qcc()?

[Katrien Baert]

Dear R-users,

I'm encoutering some problems with the qcc()-function. When there are 
more than 25 replicates per "sample" the estimate for the standard 
deviation becomes NA, implying that nothing is plotted (error message: 
"Error in plot.window(...) : need finite 'ylim' values")

On the forum I found a way to work around the error message, but this 
does not really solve the problem since the chart does not have control 
limits. The code below might make things clear.

 > data <- matrix(ncol = 30, nrow = 4)
 > for(i in 1:4){
+     data[i,] <- rnorm(30)
+ }
 > obj <- qcc(data, type = "xbar")
Error in plot.window(...) : need finite 'ylim' values
 > obj <- qcc(data, type = "xbar", ylim = c(-1, 1))
 > obj <- qcc(data[, 1:25], type = "xbar")


In the source code I find this part that calculates the standard 
deviation, and I don't see where the restriction to 25 replicates in 
implemented:

if (missing(std.dev)) {
        if (type == "xbar")
            std.dev <- "UWAVE-R"
        else if (type == "R")
            std.dev <- "UWAVE-R"
        else if (type == "S")
            std.dev <- "UWAVE-SD"
        else std.dev <- NULL
        std.dev <- do.call(sd, list(data, sizes, std.dev))
    }
    else {
        if (is.character(std.dev)) {
            std.dev <- do.call(sd, list(data, sizes, std.dev))
        }
        else {
            if (!is.numeric(std.dev))
                stop("if provided the argument 'std.dev' must be a 
method available or a numerical value. See help(qcc).")
        }
    }

I do not really see what this do.call()-function does and what is the 
function of "UWAVE-R".
Is there a reason to restrict to 25 replicates? If not, how could I 
change the function such that the standard deviation is correctly estimated?

With kind regards,

Katrien

-- 
Katrien Baert
Statistical Consultant
IOF valorisatieconsortium Stat-Gent

T 32 9 264 47 66

http://www.statgent.org/