The purpose of this causal comparative study is to determine whether the assistant principal decision-making process and their years of experience influence the advanced diploma rates in high schools on Long Island, New York. The subjects for this study were 75 assistant principals in Long Island high schools during 2016. Assistant principals' characteristics such as age, educational level, number of years as assistant principal, and gender were considered. The study incorporated a descriptive survey through purposeful sampling with specific participant criteria. The researcher developed an instrument that measures an individual's creative problem-solving and decision-making abilities. This is a multiple-choice Likert-scale instrument for adults that requires subjects to select from five possible choices, ranging from least likely to most likely, reflecting how they would respond to single-sentence problem-solving or behavioral scenarios. The raw scores reflect the frequency with which the subject would tend to initiate, respond to, or resist probing, organizing, improvising, and constructing decision behaviors. The results of this study will assist the researcher in providing useful information regarding decision-making processes. The data suggest that assistant principals' decision-making processes are influenced by the individual's length of service in that position. The results also suggest that there might be a preference for assistant principals to employ a decision-making process as it correlates with advanced diploma rates. This data was summarized by describing patterns, relationships, and connections. An inferential method was used to identify statistical differences between assistant principal decision-making processes and advanced diploma rates. There is much research on ethics, teachers, and school leadership, but only limited studies on decision-making processes and school administrators such as the assistant principal. The literature suggests that the assistant principal position is a stepping-stone to becoming the school's principal. If the assistant principal position is truly a stepping stone for future leadership, then assistant principals need to develop their professional decision-making and problem-solving skills, and have a greater understanding of the impact of those decisions, as well as the reasoning behind the implementation of those decisions that pertains to students earning advanced diplomas in high schools. [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.]