The impact of negative marking in the UPSC Preliminary Examination is significant as it discourages careless guessing among candidates. Each incorrect answer results in a deduction of 1/3 of the marks allotted to the question, which emphasizes the importance of accurate knowledge and thoughtful decision-making over random attempts. This marking scheme promotes a strategic approach to answering questions, ensuring that candidates are well-prepared and possess a sound understanding of the subject matter, ultimately affecting their chances of progression to the Mains.