The ad coefficient was developed to measure the within-group agreement of ratings. The underlying theory as well as the construction of the coefficient are explained. The ad coefficient ranges from 0 to 1, regardless of the number of scale points, raters, or items. With some limitations the measure of the within-group agreement of different groups and groups from different studies is directly comparable. For statistical significance testing, the binomial distribution is introduced as a model of the ratings' random distribution given the true score of a group construct. This method enables a decision about essential agreement and not only about a significant difference from 0 or a chosen critical value. The ad coefficient identifies a single true score within a group. It is not provided for multiple true score settings. The comparison of the ad coefficient with other agreement indices shows that the new coefficient is in line with their outcomes, but does not result in infinite or inappropriate values.