An example of how I would read Congressional Polls

[Harry Hayfield]

All data from Polling Report.com and Wikipedia:

Generic Polls on Congress in July 2006

Gallup/USA Today Jul 28-30: GOP 40% Dem 51% Others / Unsure 8%
CBS/New York Times Jul 21-25: GOP 35% Dem 45% Others / Unsure 20%
Diageo/Hotline Jul 20-23: GOP 32% Dem 48% Others / Unsure 20%
Fox/Opinion Dynamics Jul 11-12: GOP 34% Dem 42% Others / Unsure 25%
AP/Ipsos Jul 10-12: GOP 40% Dem 51% Others / Unsure 9%
Galup Jul 6-9: GOP 41% Dem 51% Others / Unsure 9%

Based on my experience of polls here in the UK, I would ignore the following polls:

CBS/New York Times Jul 21-25: GOP 35% Dem 45% Others / Unsure 20%
Diageo/Hotline Jul 20-23: GOP 32% Dem 48% Others / Unsure 20%
Fox/Opinion Dynamics Jul 11-12: GOP 34% Dem 42% Others / Unsure 25%

and therefore concentrate on the remainder which suggest an average of:

GOP 41%, Dem 51% Others / Unsure 9%

Now we know that the Others won't poll 9%, so let's look at Congress 2004:

GOP 49% Dem 46% Others 5%

Based on that let's assume that the Others stay on 5% and that the 4% who are unsure won't vote, that gives us the following change on 2004:

GOP -8% Dem +5% Others +3% which equals a swing of 6.5% to Dem.

Is that a likely interpreation, or do I need to start from scratch?

[Harry Hayfield]

So does the following sound a reasonable idea instead:

Average the Dem / GOP lead and compare with lead in 2004, then halve the swing and reduce by another half if the district has an incumbent?