Hi there, thanks for the kind notes.

In operations, the algorithms do not attempt to distinguish between any of these or definitely name a problem. This prevents the creep of (well-intentioned) overly prescriptive fixes. It instead points out when the data are indicating an issue, and in which direction the data suggest things are going off course, and a suggested fix.

With that said, the "usual suspects" are:

• changes in observation practice, particularly changes in time of observation. The US has transitioned from a nation of primarily evening observers to a nation of primarily morning observers since about WW2.
• station moves, the effect of which can be exacerbated when changes in elevation are involved; 
• changes in station environment: land use changes (removal or encroachment of trees), urbanization near a station
• changes in instrumentation or shelter: for example, there was a large swap out of temperature equipment in the 1980s for much ot the cooperative observer network, which makes up a plurality/majority of observing stations used in our national analysis.
• the intersection of two or more the above: for example, that swap-out in the 1980s influenced temperature measurement in a couple of ways, and not in the same direction. The change to electronic equipment meant wired stations, which brought them closer to buildings, generally a warming influence [relative to history], especially in the morning. But the change in shelter type from what is effectively a wooden box to a more effectively passively aspirated "beehive" type shelter had a cooling influence [relative to history], especially in the afternoon.

Each of these types of scenarios was used in a "blind test" for the algorithms - blind to the algorithms and even blind to the creators of the algorithms! - to see if the algorithms were capturing the types of changes seen, and correcting in the right direction, while minimizing harm done to "innocent" observations. The results were quite positive. While no test is perfect, the algorithms were generally correcting the signal in the proper direction.

If you're a QC geek (and it sounds like you might be!) the tests and the results are here: http://onlinelibrary.wiley.com/doi/10.1029/2011JD016761/abstract 

Deke