dear all members i have a problem with the code below, my problem in this code i want to put a high correlation between variables in group R1 and also put a high correlation between variables in group R2. after checking the correlation between variables in R1 also between variables in R2 i didn't find any correlation btween variables in R1 and between variables in R2. many thanks to Dr. William to help me to write this code and i hope to help me to complete it. many thanks to all library(psych) ords <- seq(0,1) p <- 10 N <- 1000 percent_change <- 0.9 R1 <- sim.irt(10,1000,a=3,low = -2, high=2) R2 <- sim.irt(10,1000,a=3,low = -2, high=2) R12 <- data.frame(R1$items,R2$items) #this gives you 20 items, grouped with high correlations within the first 10, and the second 10, no correlation between the first and second sets. rho <- tetrachoric(R12)$rho #find the tetrachoric correlation between the items lowerMat(rho) #show the correlations cor.plot(rho,numbers=TRUE) #show a heat map of the correlations R1 <- as.data.frame(replicate(p, sample(ords, N, replace = T))) R2 <- as.data.frame(replicate(p, sample(ords, N, replace = T))) rho <- tetrachoric(R12)$rho #find the tetrachoric correlation between the items lowerMat(rho) #show the correlations R1 R2 [[alternative HTML version deleted]]