userfriendlyscience.com
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The userfriendlyscience R package offers a number of functions to make computing a number of reliability estimates very easy. Please see http://userfriendlyscience.com for installation instructions.

The papers explaining these functions are Open Access, and in the European Health Psychologist: see this page for the table of contents, and this direct link to the paper about Cronbach's Alpha, Omega, and the Greatest Lower Bound, and this paper about test-retest reliability. This open access paper in Health Psychology Review about factor analysis, which provides guidelines for authors, reviewers and editors regarding scale quality and what should be reported, can also be useful.

Some examples of these functions:
### This will prompt the user to select an SPSS file (see the article for ### an explanation, or just use ?scaleStructure in R to see the help). scaleStructure(); ### Load data from simulated dataset testRetestSimData (which ### satisfies essential tau-equivalence). data(testRetestSimData); ### Select some items in the first measurement exampleData <- testRetestSimData[2:6]; ### Use all items to compute the glb and omega scaleStructure(dat=exampleData, ci=FALSE); ### Use a selection of two variables scaleStructure(dat=exampleData, items=c('t0_item2', 't0_item4'), ci=FALSE); ### Note that 'ci = FALSE' can be omitted to compute confidence intervals ### for scales of three items or more. ### It also contains functions to compute the test-retest reliability. ### The first column is the true score, so it's excluded in this example. exampleData <- testRetestSimData[, 2:ncol(testRetestSimData)]; ### Compute test-retest alpha coefficient testRetestReliability(exampleData);

If you have any questions, you can contact me through http://behaviorchange.eu.