Sampling bias can be a concern in any measurement. While Alexa and Hitwise do have very large samples that would help guard against bias, Google provides an even larger pool of users. Using Google Site Trends we see once again the gap between the usage of the candidate sites

A deeper drill down into this Google-based trending reveals some interesting points. First, we see that when looking at the data on a day-to-day basis, there is a clear pinch in the trend lines on Sept. 2 that is normalized out in the month-to-month view. This validates what we saw earlier in larger graphs when the Republican convention was finally in full swing.

Google data also shows that much of this traffic comes from the large population states. The lists below show the top 10 states, by traffic, for each candidate's site. It is interesting to see that some of the so-called "battleground" states (Florida, Virginia, Pennsylvania, Colorado) are represented at different positions in the two lists. What is most notable, however, is that you can see a consistent lead for Obama (represented by the blue horizontal bars) regardless of which candidate's top-10 ranking you use. That is, his site leads not only in all 10 of the states in which he does best, but also in the McCain site's 10 highest-traffic states as well.

Raw traffic to a site may of course come from those against a candidate, as well as from supporters. However, features like "users also visited" on the sites like Alexa, Google and Quantcast suggest that this is likely not a major factor in the traffic rankings. Instead, they indicate that visitors to a given candidate's site also tend to spend time on other sites with political orientations similar to the candidate's own. Thus, visitors to Obama's site tend also to visit popular liberal/progressive sites, while visitors to McCain's site tend to favor well-known conservative sites. If the opposite traffic effect were sizable, there would likely be a more mixed list of also-visited sites.

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