Sentence:  The authors propose using <WSD>polychoric</WSD> correlations in order to estimate the correlation matrix before using PCA.[Principal Component Analysis] <WSD>Polychoric</WSD> PCA assumes that the observed ordinal variable has an underlying continuous variable and uses maximum likelihood to calculate how that continuous value would have to be split up in order to produce the observed data. 
Meaning of the word marked with <WSD> tags:  Of or pertaining to the relationship between two latent variables, each assumed to have a normal distribution and associated with an ordinal variable.