These include artificial changes in the record due to the replacement of thermometers or changes in observing practices, such as the change from imperial to decimal units in the middle of last century. (bom.gov.au)
One method suggests comparing the data from more reliable weather stations that have undergone the least amount of known artificial changes to determine a «regional expectation.» (landscapesandcycles.net)
Originally, only documented cases were adjusted, however the current procedure used by NOAA / NCEI applies an automated system that uses systematic comparisons with neighboring stations to deal with undocumented instances of artificial changes. (data.giss.nasa.gov)