The CRU has refused to release the raw weather station data and its processing methods for inspection - except to hand-picked academics - for several years. Instead, it releases a processed version, in gridded form. NASA maintains its own (GISSTEMP), but the CRU Global Climate Dataset, is the most cited surface temperature record by the UN IPCC. So any errors in CRU cascade around the world, and become part of "the science".
Professor Phil Jones, the activist-scientist who maintains the data set, has cited various reasons for refusing to release the raw data. Most famously, Jones told an Australian climate scientist in 2004:
Even if WMO agrees, I will still not pass on the data. We have 25 or so years invested in the work. Why should I make the data available to you, when your aim is to try and find something wrong with it.