When and how to call election results is both nebulous and highly contentious. In our case, we’re erring on the side of caution at the blog and following the lead of the Assocated Press with updates and final projections. The AP, while hardly infallible, is known for taking a more cautious approach than many other outlets. That’s also why we deliberately decided not to begin our on-air coverage until after polls closed in Illinois at 7pm CT. Here’s the latest explanation from NPR Senior Washington Editor Ron Elving on how they’re approaching this:

This is some further guidance for you on how NPR will be calling the election results tonight. This guidance is particularly of relevance if the exit polls and early results point to a big win either way in the Presidential race.

Already, some of the early findings of exit polls have made their way onto the Internet.

NPR has subscribed to the National exit poll compiled for all the major media organizations, along with polls from five key states: Virginia, Ohio, North Carolina, Pennsylvania and Florida. We will use data from these polls to inform our decision on projecting winners, as we have done in previous elections.

If Senator Obama or Senator McCain takes leads in some or all of those five key states, we may see some websites and media organizations calling the entire election on the strength of a likely pattern.

NPR has said we will NOT call individual states before their polls close, including California at 11pmEST.

NPR has said we will NOT call the national outcome while the polls are open in the lower 48 states.

But we are also cognizant that we need to reflect the trends of the early results and give listeners the facts of what calls we have made and what results are to hand ”¦ and what those may portend in the final analysis.

So there you have it.