C. Investigation of Dimensionality questions

Q1. Can I use other statistics software to compute tetrachoric correlations?

A1: Of the three most popular statistical analyses packages, SYSTAT is the easiest one to compute tetrachoric correlations. SAS's PROC FREQ procedure can compute tetrachoric correlations. You may download a macro provided by SAS at http://support.sas.com/ctx/samples/index.jsp?sid=512, or refer to a FAQ provided by UCLA's Academic Technology Services' web site at http://www.ats.ucla.edu/stat/sas/faq/tetrac.htm. While SPSS does not have built-in syntax to compute tetrachoric correlations, an SPSS macro, r_tetra, is available for computing tetrachoric correlation.

Q2. While investigating dimensionality, why am I getting multiple factors?

A2: At C. Investigation of Dimensionality, we suggested using Principle Axis Factoring (PAF), rather than Principal Components Analysis (PCA), to determine the number of factors in a measure. In SYSTAT, the default factor analysis method is PCA, not Iterated Principle Axis (IPA). You may refer to IRT\Investigating Unidimensionality for Dichotomous Data.ppt for more details.