From a recent posting at realclimate:
Their [M&M] second criticism is of the statistic employed by MBH98 as diagnostic of statistical skill, the "Reduction of Error" or "RE" (note that this statistic was favored as a skill diagnostic in prominent recent studies by Cook et al (2004) and Luterbacher et al (2004) in Science)… MM instead promote the use of a simple linear correlation coefficient ("r") in its place.
Obviously we do not such thing. We’ve advocated the use of more than one verification statistic, showing in our GRL article that spuriously high RE statistics can be generated by data mining procedures, such as the erroneous MBH98 PC procedure. What is ironic in Mann’s citation of Cook as authority that paleoclimatologists need not provide a range of verification statistics is that Cook et al.  actually provide a suite of verification statistics, headed by the R2 statistic, which Mann et al. now object to. Cook et al.  states:
The calibration and verification statistics used to assess the goodness-of-fit and validity of the PDSI reconstructions are (i) the calibration period coefficient of multiple determination or regression R2 (CRSQ), (ii) the verification period square of the Pearson correlation coefficient or r2 (VRSQ), (iii) the reduction of error (RE), and (iv) the coefficient of efficiency (CE) …When these statistics are calculated for the 103–grid point West regional average PDSI reconstruction, CRSQ, VRSQ, RE, and CE are 0.86, 0.73, 0.78, and 0.72, respectively, for the most highly replicated post-1800 period of the reconstruction, based on a median of 41 tree-ring predictors per grid point reconstruction, and 0.68, 0.54, 0.64, and 0.53, respectively, when based on the smallest subset of tree-ring predictors available at the start of each grid point reconstruction (a median of 2 per grid point) (fig. S3).
Some online discussion is at the SI to Cook et al. . In this case, Cook et al. show a higher R2 than RE. This is obviously a completely different situation than MBH98 where there are spuriously high RE values from data mining and insignificant R2 values in the AD1400 step (estimated at about 0.0). A point of follow-up is that Cook et al.  is a study of drought in the U.S. from tree rings. Cook et al. report the use of 835 tree ring chronologies, of which 602 are in the "West". They report the use of 17 chronologies in AD800.
I requested a listing of the sites from Connie Woodhouse of NOAA, one of the co-authors, to try to see if these sites overlapped with the sites used in MBH98 as "temperature" proxies. The map in Figure S2 of the SI sure looks like it overlaps with MBH98. I’m into the typical song-and-dance for the simplest such request. Woodhouse says that I have to get the information from Cook. I’ve had no luck getting data from Cook – I’ve been trying to get their updated Gaspé information (which does not have a hockey stick shape) for nearly a year; they’ve refused to provide. I’ve tried to get the exact location of the Ste Anne River, Gaspé site to commission re-sampling; they’ve refused to provide. I suspect that it will be a long time to get the list of sites in this study.