The National Election Pool — a joint venture of the major television networks and The Associated Press — has not announced the states that won’t be included, but the decision is sure to cause some pain to election watchers across the country.

Voters in the excluded states will still be interviewed as part of a national exit poll, but state-level estimates of the partisan, age or racial makeups of electorates won’t be available as they have been since 1992. The lack of data may hamper election night analyses in some states, and it will almost certainly limit post-election research for years to come.

A growing number of voters casting early ballots has added to the complexity of carrying out surveys in 50 states, the District of Columbia and nationally. In more and more states it has become crucial to supplement in-person precinct polling with relatively costly telephone interviews in order to achieve representative samples.