The study established the validity and reliability of a weighted individual performance-based assessment tool within the utility scope of middle school orchestral strings. The following research questions guided this study: 1. What specific string-playing behaviors and corresponding criteria validate a weighted individual performance-based assessment tool for middle school orchestral strings? 2. What are the psychometric properties of the weighted individual performance-based assessment tool in authentic situations? For Research Question 1, the expert panel and I were able to 100% mutually agree on 10 string-playing behaviors: tempo, rhythm, tone, pitch, intonation, technique, bowing, dynamics, phrasing, and posture that created the DISAT. Being interdependent, these string-playing behaviors are relevant because they encompass every necessary facet of orchestral string performance (Zdzinski & Barnes, 2002). According to Zdzinski and Barnes (2002), an orchestral string performance assessment must evaluate each facet of a participant's playing ability to rate the overall musicianship. Bergee and Rossin (2019) stated in their research that it is important to have various aspects of a performance utilized in a musical assessment. The DISAT obtained reliability of 0.872 by having enough variance between raters in the authentic situation. Linacre (2015) stated that reliability greater than 0.8 is acceptable to distinguish separation between raters. Combined with the expert panel's 100% mutual agreement on content validity, this proved the DISAT to be a valid and reliable assessment tool for individual performance-based orchestral strings assessment (AERA, APA, & NCME, 2014). The DISAT can be utilized by districts and middle school orchestral string music teachers in North Carolina. Being a consistent, objective tool, the DISAT can standardize our approach to middle school orchestral string music education assessment (AERA, APA, & NCME, 2014). The data collected by the DISAT could easily track the musical progression of students while giving opportunities for constructive, purposeful feedback. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]