From Alpha to Omega: Estimation of Ordinal Reliability. A Practical Guide.

The present study takes a tour of the concept of reliability as one of the fundamental psychometric properties in Classical Tests Theory. The concept and its different practical applications are developed to investigate the degree of reliability of a measuring instrument. Focusing on the calculation of internal consistency from alpha and omega as the most used coefficients and the importance of calculating these coefficients by using polychoric correlation matrices (PCM). The main objective is to present a guide in Spanish for the calculation of ordinal reliability coefficients using the R/Rstudio program. Providing an example at the empirical level that shows the relevance of calculating this type of coefficients for calculating the reliability of an instrument. Using a sample of 266 adults between the ages of 18 and 63 years (M = 31.91 SD = 11.50), the version adapted to Argentina of the Beck Anxiety Inventory and the Emotional Regulation Questionnaire were administered. In this way, coefficients are exposed to estimate the reliability of the instruments that account for their advantages and disadvantages, performing the calculation using MCP, Pearson's correlation matrix and Pearson's covariance matrix. From the results, it is evident that the calculation using MCP provided higher degrees of reliability compared to the calculation using the other two matrices. This document is expected to be of importance to researchers unfamiliar with R.