NCEP-NCAR (R1) is the original reanalysis effort. It uses a frozen global state-of-the-art global data assimilation system (as of 11 January 1995). The original database was enhanced (additional, quality checked datasets) by NCAR's Data Support Section. Originally planned to span 1957-96 ("40-Year Reanalysis Project"), it was extended back to 1948 and continues to this day.
Kalnay et al (1996) provide a four class partitioning of the R1 output analyzed variables. Specifically:
National Center for Atmospheric Research Staff (Eds). Last modified 05 Aug 2015. "The Climate Data Guide: NCEP-NCAR (R1): An Overview." Retrieved from https://climatedataguide.ucar.edu/climate-data/ncep-ncar-r1-overview.