CHAPTER 1: Introduction: The most common craniofacial congenital anomaly is the cleft lip and/or palate (CLP). The CLEFT-Q is the first condition-specific comprehensive patient reported outcome instrument (PROM) for patients with CLP. Other measures used in assessment of patients with CLP are Child Oral Health Impact Profile (COHIP) and Cleft Hearing, Appearance and Speech Questionnaire (CHASQ). The development and validation of the CLEFT-Q have been completed in three phases. In phase I, 138 patients with CLP from six countries were interviewed, and data were used to form 13 scales measuring appearance, facial function and health-related quality of life (HR-QOL). In phase II, scales were field-tested internationally with 2434 patients to examine reliability and validity as well as develop a common scoring algorithm for international use. Phase III, the focus of this thesis, aimed to examine further construct validity and responsiveness of the CLEFT-Q scales. CHAPTER 2: Methods: Patients were recruited at six cleft centres in Canada, USA and UK between January 2018 and October 2019. The sample included patients aged 8-29 seeking rhinoplasty, orthognathic, cleft lip scar revision and alveolar bone graft (ABG) operations. Before and six months after surgery, participants were asked to complete the CLEFT-Q scales relevant to their operation and two other PROMs frequently used in cleft research, i.e., COHIP and CHASQ. Cross-sectional construct validity was examined by testing prespecified hypotheses about expected relationships between CLEFT-Q, CHASQ and COHIP instruments. Internal responsiveness was examined using the distribution-based method. Data were analysed using paired sample t-tests and calculation of effect sizes (ESs) and minimally important differences (MIDs). CHAPTER 3: Results: Examination of cross-sectional construct validity of the CLEFT-Q scales using the COHIP and CHASQ subscale resulted in 39/53 (74%) hypotheses having been supported by the results. The required sample size to examine responsiveness using the anchor-based approach was not reached. Assessment of internal responsiveness using the distribution-based approach demonstrated that the appearance scales were highly responsive to change following cleft-specific surgeries, with statistically significant results and ESs ranging from 0.4 (small) to 1.8 (large). Change on the CLEFT-Q HR-QOL scales was not statistically significant. As predicted, the ESs on scales measuring facial aspects most affected by rhinoplasty and orthognathic surgeries were larger than the ESs on scales measuring facial aspects least affected by these surgeries. MIDs for each scale in each operation were determined. CHAPTER 4: Discussion: Assessment of cross-sectional construct validity demonstrated that CLEFT-Q performs as it was intended when compared with other similar measures (i.e., CHASQ and COHIP). The CLEFT-Q appearance scales were responsive to change following rhinoplasty, orthognathic and cleft lip scar revision operations. As predicted, the CLEFT-Q appearance scales did not detect change following the ABG operation as this operation does not result in visible difference. As hypothesized, the CLEFT-Q HR-QOL scales were less responsive to change than appearance scales as HR-QOL is a more distal construct than appearance in relation to the cleft-related surgeries performed. The preliminary MIDs estimated by the distribution-based approach should be confirmed in studies with diverse CLP populations and larger sample sizes using the anchor-based approach. The findings of this phase III study build on the results of another CLEFT-Q validation study, which demonstrated the ability of the CLEFT-Q scales to detect differences between groups with varying surgical status, i.e., need surgery, have had surgery and never needed surgery. CHAPTER 5: Conclusion: Cross-sectional construct validity of the CLEFT-Q scales was supported by most prespecified hypotheses. The CLEFT-Q scales were found to be responsive to change. MIDs were determined. The results of this phase III study should be confirmed in a larger and more culturally diverse patient population. Future studies to examine reproducibility and measurement error as well as external responsiveness of the CLEFT-Q scales may be beneficial to add to the psychometric evaluation process. / Thesis / Master of Science (MSc)