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by Kilem L. Gwet, Ph.D.

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Calculating the AC2 Coefficient with AgreeStat
Posted: Monday, May 2, 2011

 

On a number of occasions, I was asked whether the agreement coefficient ac2 coefficient could be calculated with the AgreeStat Excel VBA program. The answer to this question is yes, AgreeStat can compute the agreement coefficient ac2
coefficient along with its standard error. In section 4.7 (page 80) of Gwet (2010), I indicate that agreement coefficient ac2 is equivalent to the weighted ac1 agreement coefficient based on the quadratic weights. Therefore, you will obtain agreement coefficient ac2 with AgreeStat by calculating the weighted ac1 agreement coefficientafter selecting the quadratic weights. Figure 1, shows in the case of 2 raters how Quadratic weights could be selected.

To compute Quadratic weights, AgreeStat uses equation 4.20 in Gwet (2010). Note that, category values are numeric, alphabetic, or alphanumeric. If the category values are numeric, these numeric values are used as x-values in the equation. Otherwise, the categories are assigned sequential x-values 1,2,3 until the total number of categories, which are later used in the equation.

agreestat

Figure 1. Calculating the agreement coefficient ac2 Coefficient for 2 Raters

References:
Gwet, K.L. (2010). Handbook of Inter-Rater Reliability (2nd Edition), Advanced Analytics, LLC

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