psych :: alpha不返回均值，sd

Question

The psych package contains a function ?alpha which calculates test reliability and some item statistics. When fed raw data (a data.frame with binary values for correct/incorrect answers), it returns, among other things, mean and st. dev. for each item. However, sometimes it doesn't, and only provides item-whole correlation.

Why is that?

The docu states that mean and sd are only calculated "For data matrices [...]", and that x is "A data.frame or matrix of data, or a covariance or correlation matrix". But how does it know whether I'm giving it raw data or a correlation matrix?

Answer 1

The alpha command of psych detects if the input x is a correlation matrix using the isCorrelation command. Let consider the dataset given in this tutorial :

datafilename <- "http://personality-project.org/R/datasets/extraversion.items.txt" 
items <- read.table(datafilename,header=TRUE)    
df <- with(items, data.frame(q_262 ,q_1480 ,q_819 ,q_1180 ,q_1742 )) 

The object df is a data.frame whose columns represent the responses to five items. The command isCorrelation (used inside alpha ) correctly detect that this is not a correlation matrix:

library(psych)
isCorrelation(df)
[1] FALSE

If we calculate the correlation matrix of df and pass to isCorrelation , we get again a correct answer:

mtx <- cor(df)
isCorrelation(mtx)
[1] TRUE

Looking inside isCorrelation one can see that a correlation matrix is defined as an object which is not a data.frame (hence, is a matrix ) and which is symmetric:

isCorrelation <- function (x) 
{
    return(!is.data.frame(x) && isSymmetric(unname(x)))
}