Modifying the C Index for Use With Holland Codes of Unequal Length

The concept of congruence between person and occupation lies at the core of Holland’s (1997) theory of career types. The C index is arguably the best available method for comparing the congruence of two Holland code profiles. The C index reflects the theorized hexagonal structure of the Holland RIASEC model, is sensitive to code ordering, and is simple to calculate. However, the C index is formulated to only make comparisons between Holland code profiles three letters in length. Although this is consistent with the instrumentation and supporting materials developed by Holland and his colleagues, it is inconsistent with both the Strong Interest Inventory (SII) and the Occupational Information Network (O*NET), each of which assigns Holland codes of one to three letters. Consequently, the C index cannot be easily used with either the SII or the O*NET. Moreover, the authors argue that it is arbitrary to always calculate congruence using Holland codes three letters in length and that congruence should only be calculated using those Holland types that are clearly salient in the profiles being compared. The modifications to the C index proposed in this article allow comparisons between Holland code profiles of unequal lengths and/or of less than three letters in length and retain the desirable properties of the original C index: reflection of the hexagonal structure, sensitivity to order, and simplicity of calculation.