Sea Ice Index images depict ice cover and trends in ice cover in the Arctic and Antarctic oceans. Sea Ice Index data files tabulate ice extent in numbers. The images and data are produced in a consistent way that makes the Index time-series appropriate for use when looking at long-term trends in sea ice cover. Both monthly and daily products are available. However, monthly products are better to use for long-term trend analysis because errors in the daily product tend to be averaged out in the monthly product and because day-to-day variations are often the result of short-term weather. Sea Ice Index products are derived from two data sets: the Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product, NSIDC-0081) and the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (GSFC product, NSIDC-0051). These satellite passive microwave-derived data sets are used to generate the daily and monthly images and numbers that comprise the Sea Ice Index record of sea ice extent and concentration from November 1978 to present. Information on the accuracy and precision of passive microwave-derived sea ice concentration products can be found in this documentation as well as in the documentation for the NRTSI and GSFC products.