rqcc

How can I plot multiple control limits in an EWMA control chart with qcc?


I can plot an EWMA control chart with 3 sigma limits (nsigmas=3). Can anyone help me with plotting additional control lines on the same chart such as 1 and 2 sigma limits?

The only way I can think is to create separate qcc objects with each of these limits, somehow extract their values, and then plot them onto the EWMA chart. Surely there's a simpler way?

library(qcc)
LRR <- c(-0.1, -0.1, -0.09, -0.07, -0.27, -0.18,
-0.8, -0.86, -0.82,  0.01,  0.02)

q1 <- ewma(LRR, center =  -0.3, std.dev = 0.1 , lambda = 0.2,plot=F)

plot(q1, add.stats = F, label.limits = c("LCL", "UCL"), 
     xlab="Group", ylab= "LRR", ylim=c(-1.0,0.5),nsigmas = 3)

Now I'd just like to add 1 & 2 sigma control limits.

Thanks.


Solution

  • Thank you so much Paul. Your answer is certainly more sophisticated than mine was!

    I ended up solving it by;

    'escalc' to find the LRR, then

        rma(yi=LRR,vi=LRR_var)
    

    est = 'estimate' from this rma

        q1 <- ewma(LRR, center =  est,  lambda = 0.2,labels=year,plot=F)
        LCL1 <- est - q1$sigma
        LCL2 <- est - 2*q1$sigma
        UCL1 <- est + q1$sigma
        UCL2 <- est + 2*q1$sigma
        LCL3 <- est - 3*q1$sigma
        UCL3 <- est + 3*q1$sigma
    

    Then using plot

        abline(h=center,lty=1)
        lines(spline(1:4,LCL1,n=10), lty = "dotted", col = "grey")
        lines(spline(1:4,UCL1,n=10), lty = "dotted", col = "grey")
        lines(spline(1:4,LCL2,n=10), lty = "dotted", col = "black")
        lines(spline(1:4,UCL2,n=10), lty = "dotted", col = "black")
        lines(spline(1:4,LCL3,n=10), lty = "longdash", col = "black")
        lines(spline(1:4,UCL3,n=10), lty = "longdash", col = "black")