Even a fairly calm spell in the polling, like the last couple of days, can give people opportunities to see what they want in the data.
The most egregious form of this is cherry-picking the three or four polling results that you like best for your candidate. A vast majority of the time, you can find a couple that are favorable for your side. …
There is a more subtle form of bias, however, that a lot more of us are prone to. That bias is to look at all the data — except for the two or three data points that you like least, which you dismiss as being “outliers.” [cont.]
Nate Silver, FiveThirtyEight (NYT)