The first truly quantitative reconstruction of Northern Hemisphere temperatures had been published in 1993 by Bradley and Phil Jones, but it and subsequent reconstructions compiled averages for decades, covering the whole hemisphere. Mann wanted temperatures of individual years showing differences between regions, to find spatial patterns showing natural oscillations and the effect of events such as volcanic eruptions. Sophisticated statistical methods had already been applied to dendroclimatology, but to get wider geographical coverage these tree ring records had to be related to sparser proxies such as ice cores, corals and lake sediments. To avoid giving too much weight to the more numerous tree data, Mann, Bradley and Hughes used the statistical procedure of principal component analysis to represent these larger datasets in terms of a small number of representative series and compare them to the sparser proxy records. The same procedure was also used to represent key information in the instrumental temperature record for comparison with the proxy series, enabling validation of the reconstruction. They chose the period 1902–1980 for calibration, leaving the previous 50 years of instrumental data for validation. This showed that the statistical reconstructions were only skillful (statistically meaningful) back to 1400.