Thanks!
Unfortunately now after omitting the ">" symbol in the code:
cor(a_LSshort[c("WLLP", "WRT", "Cody", "Lernen", "mob_z", "INT_z", "EXT_z")]) # correlation matrix
I got the following error:
Error in cor(a_LSshort[c("WLLP", "WRT", "Cody", "Lernen", "mob_z", "INT_z", :
supply both 'x' and 'y' or a matrix-like 'x'
Would you know how to overcome this?