Is one more biased?
Is one more accurate?
I examine 5 large polling companies (separating Zogby and Zogby Interactive) and how they did in rating Senate races recently.
Details below the fold
I went to The Pollster and looked at their 'hot' Senate races. For each, I compared various polling companies' numbers to the average for the last 10.
There are two things you want in such numbers: Low bias and low variance. Bias means a company favors R or D. Variance is a measure of how tightly together the numbers are.
I measure bias via the mean - a mean close to 0 indicates little bias. A positive number favors D. The averages don't sum to 0 bcs. not all polls in the last 10 are included
What did I find?
Gallup mean 2.5 variance 6.28
Mason Dixon mean 0 variance 5.55
Rasmussen 0.53 9.76
SUSA 2 12
Zogby -1.12 22.12
Zogby interact -1 10
So, at least for Senate races, the best company is Mason Dixon, in terms of both bias and variance. Worst is harder to say, but Zogby looks very bad (interestingly, their phone polls are WORSE than interactive) as does Gallup