glb.algebraic {psych} | R Documentation

## Find the greatest lower bound to reliability.

### Description

The greatest lower bound solves the “educational testing problem". That is, what is the reliability of a test? (See `guttman` for a discussion of the problem). Although there are many estimates of a test reliability (Guttman, 1945) most underestimate the true reliability of a test.

For a given covariance matrix of items, C, the function finds the greatest lower bound to reliability of the total score using the csdp function from the Rcsdp package.

### Usage

    glb.algebraic(Cov, LoBounds = NULL, UpBounds = NULL)

### Arguments

`Cov` | A p * p covariance matrix. Positive definiteness is not checked.
`LoBounds` | A vector L = (l1 ... lp) of length p with lower bounds to the diagonal elements x_i. The default l=(0, . . . , 0) does not imply any constraint, because positive semidefiniteness of the matrix C0 + Diag(x) implies 0 ≤ xi.
`UpBounds` | A vector u =(u1, . . . , up) of length p with upper bounds to the diagonal elements xi. The default is u = v.

### Details

If C is a p * p-covariance matrix, v = diag(C) its diagonal (i. e. the vector of variances v_i = c_{ii}), C0 = C - Diag(v) is the covariance matrix with 0s substituted in the diagonal and x = the vector (x1, . . . , xp) the educational testing problem is (see e. g., Al-Homidan 2008)

(Sum i = 1 to p xi) -> min

s.t.

C0 + Diag(x) >= 0

(i.e. positive semidefinite) and xi ≤ vi, i = 1 ..., p. This is the same as minimizing the trace of the symmetric matrix

C0 + Diag(x)

s. t. C0 + Diag(x) is positive semidefinite and xi ≤ vi.

The greatest lower bound to reliability is

(sum cij (i \ne j) + sum xi )/ sum cij

Additionally, function glb.algebraic allows the user to change the upper bounds xi ≤ vi to xi ≤ ui and add lower bounds li ≤ xi.

The greatest lower bound to reliability is applicable for tests with non-homogeneous items. It gives a sharp lower bound to the reliability of the total test score.

Caution: Though glb.algebraic gives exact lower bounds for exact covariance matrices, the estimates from empirical matrices may be strongly biased upwards for small and medium sample sizes.

glb.algebraic is wrapper for a call to function csdp of package Rcsdp (see its documentation).

If Cov is the covariance matrix of subtests/items with known lower bounds, rel, to their reliabilities (e. g. Cronbachs α), LoBounds can be used to improve the lower bound to reliability by setting LoBounds <- rel*diag(Cov).

Changing UpBounds can be used to relax constraints xi ≤ vi or to fix xi-values by setting LoBounds[i] < -z; UpBounds[i] <- z.

### Value

`glb` | The algebraic greatest lower bound
`solution` | The vector x of the solution of the semidefinite program. These are the elements on the diagonal of C.
`status` | Status of the solution. See documentation of csdp in package Rcsdp. If status is 2 or greater or equal than 4, no glb and solution is returned. If status is not 0, a warning message is generated.
`Call` | The calling string

### Author(s)

Andreas Moltner
Center of Excellence for Assessment in Medicine/Baden-Wurttemberg
University of Heidelberg

William Revelle
Department of Psychology
Northwestern University Evanston, Illiniois
http://personality-project.org/revelle.html

### References

Al-Homidan S (2008). Semidefinite programming for the educational testing problem. Central European Journal of Operations Research, 16:239-249.

Bentler PM (1972) A lower-bound method for the dimension-free measurement of internal consistency. Soc Sci Res 1:343-357.

Fletcher R (1981) A nonlinear programming problem in statistics (educational testing). SIAM J Sci Stat Comput 2:257-267.

Shapiro A, ten Berge JMF (2000). The asymptotic bias of minimum trace factor analysis, with applications to the greatest lower bound to reliability. Psychometrika, 65:413-425.

ten Berge, Socan G (2004). The greatest bound to reliability of a test and the hypothesis of unidimensionality. Psychometrika, 69:613-625.

### See Also

For an alternative estimate of the greatest lower bound, see `glb.fa`. For multiple estimates of reliablity, see `guttman`

### Examples

    Cv<-matrix(c(215, 64, 33, 22,
                  64, 97, 57, 25,
                  33, 57,103, 36,
                  22, 25, 36, 77),ncol=4)

    Cv                    # covariance matrix of a test with 4 subtests
    Cr<-cov2cor(Cv)       # Correlation matrix of tests
    if(!require(Rcsdp)) {print("Rcsdp must be installed to find the glb.algebraic")} else {
     glb.algebraic(Cv)     # glb of total score
    glb.algebraic(Cr)      # glb of sum of standardized scores

     w<-c(1,2,2,1)         # glb of weighted total score
     glb.algebraic(diag(w) %*% Cv %*% diag(w))
    alphas <- c(0.8,0,0,0) # Internal consistency of first test is known

    glb.algebraic(Cv,LoBounds=alphas*diag(Cv))

                          # Fix all diagonal elements to 1 but the first:

    lb <- glb.algebraic(Cr,LoBounds=c(0,1,1,1),UpBounds=c(1,1,1,1))
    lb$solution[1]        # should be the same as the squared mult. corr.
    smc(Cr)[1]
    }

[Package psych version 1.6.9 Index]