Reliability sits at the core of instrument development within the Classical Test Theory framework during this period. The introduction of Cronbach's alpha provided a practical lower bound on reliability and linked it to tau-equivalence and parallel forms, guiding scale construction and consistency assessment. Researchers also advanced mood assessment and depressive-illness measurement through brief, reliable checklists and structured scales, while investigations into Likert scale item formats informed decisions about response categories and respondent burden. Methodologically, intercorrelations, factor analysis, and Varimax rotation were employed to build interpretable scales and clarify when composite scores are valid under congeneric and tau-equivalent models.