Pseudoproxies
The skill of algorithms used to combine proxy records into an overall hemispheric temperature reconstruction may be tested using a technique known as "pseudoproxies". In this method, output from a climate model is sampled at locations corresponding to the known proxy network, and the temperature record produced is compared to the (known) overall temperature of the model. Some methods of combining the records, in particular least squares, perform poorly.
Read more about this topic: Proxy (climate)