Reliable sources for statistical data in UPSC preparation include government databases, such as the Census of India and the National Sample Survey Organization (NSSO), which provide official information on demographics and socio-economic conditions. Reports from reputable organizations like the World Bank, UNICEF, and National Statistical Office (NSO) offer credible insights into various sectors. Academic journals and publications from think tanks also present comprehensive analyses. Utilizing these sources helps aspirants build a strong foundation of evidence-based knowledge for their examination papers.