A psychometric analysis was conducted using the nominal response model under the item response theory framework to construct the Positive Family Relationships scale. Using data from the Fullerton Longitudinal Study, this scale was constructed within a long-term longitudinal framework spanning middle childhood through adolescence. Items tapping this construct were completed annually by mothers when children were aged 9 to 17 years. The scale measures a construct customized for each age with uniquely informative items consisting exclusively of discriminating response categories as evaluated using the nominal response model. By using longitudinal data, the study is innovative in implementing the method of test equating to a psychological construct across nine assessments spanning 8 years. Future research pertaining to validity is discussed.