We realise having everyone declare anomalies with reference to different base - periods is frustrating and we try to maintain at least similar base reference periods for data used in the graphs. (junksciencearchive.com)
When scientists compare average sea ice conditions between years, they often use a 30 - year reference period of 1981 to 2010. (nsidc.org)
Each day's heating degree days are summed to create a heating degree day measure for a specified reference period. (eia.gov)