Since a fair number of folks seem to be scared by recent polls, here's an interesting poll from CNN today. It has Obama leading Romney 52% to 46% (yikes!). But (see page 21) Romney leads among independents 54% to 40%. If you assume that each respondent is either D or I or R, you can use these numbers to back out, for each level of I, the percentages of D and R in the poll. (It's two linear equations in two unknowns, once you specify I.)
If I = 20%, you compute D = 44.63% and R = 35.37% (using the equation .52 = .97D + .4I + .02R), or a 9.26% D over R.
If I = 40% instead, you compute D = 36.63% and R = 23.37%, or an 13.26% difference.
For I between 20% and 40%, the difference is between 9.26% and 13.26%---the larger is I, the larger is the computed value of D minus R.
Of course, we don't know I. (CNN doesn't supply the sample statistics.) But we can compute for each I the level of support for Romney given the computed D and R, and the closest we come to 46% is when I = 20%, for a "lean-Romney" number of 45.7%. (If we went lower with I, we could come closer. At I = 40%, the computed "lean-Romney" is 44.3%.)
So, the best guess I can make is that CNN's sample is I = 20%, D = 45% (or so), and R = 35%.
Conclusion #1: CNN has some very interesting sampling procedures. This strikes me as propaganda and not a serious exercise in polling.
Conclusion #2: Chill out. The polls for public consumption can't be taken seriously this far away from the real polling day.