The purpose of this causal-comparative study was to examine whether differences exist in the mathematics achievement of fifth grade gifted students based on the instructional delivery model used for mathematics instruction, cluster or collaborative, as defined by the Georgia Department of Education. The content area of mathematics, an area susceptible to underachievement among gifted learners, was investigated using archival data from a sample of 67 participants from rural Southwest Georgia over three academic years. The STAR Math assessment and the Georgia Criterion-Referenced Competency Test (CRCT): Math assessments were used to measure overall mathematics achievement. The subscales on the CRCT were used to measure mathematical proficiency in numbers and operations, measurement, geometry, algebra, and data analysis. A one-way analysis of variance (ANOVA) was used on the data from the STAR Math assessment to analyze mathematics achievement. A multivariate analysis of variance (MANOVA) was used on the scale score data from the CRCT to analyze overall mathematics achievement. Results from the ANOVA on the STAR Math assessment data revealed no significant difference between comparison groups. Results from MANOVA on the CRCT revealed a significant main effect difference on overall mathematics achievement between comparison groups. The posthoc pairwise comparisons revealed significant differences on the subscales of geometry and algebra. No significant differences were found on the subscales of numbers and operations, measurement, and data analysis and probability. Suggestions for further experimental research are included. [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/en-US/products/dissertations/individuals.shtml.]