Principal components were retained if their eigenvalues were greater than 1.0, a threshold that is commonly used as a cut-off to identify meaningful patterns. The resulting components were interpreted based on the variables with loadings above 0.3.Moreover, the Kaiser–Meyer– Olkin criterion was used as a measure of variable (component) inter-correlation and used as an indicator of internal consistency, which is an important attribute of an index that intends to measure a single underlying concept (in this case successful aging).