Excuse me while I copy the context/explanation for the below graphs from earlier posts in different threads..
It's no news that I'm a big fan of pollster.com, and specifically, its lovely and insightful graphs, in which all national and state polls are tracked, trendlines and all.
For example, these ones on the
Democratic race in Iowa, the
Republican race in Iowa, the
Democratic race in New Hampshire, and the the
Republican race in New Hampshire.
But sometimes I'm a bit frustrated by them. The pollster.com trendlines are extremely cautious. With good reason, but still. Even when new results come in that diverge from recent ones, mostly the trend line is just made a little more or less steep; actual "bends" or turns in the trend are made only once a three months or so at most.
Again, all for very sound methodological reasons. But I'm following these races and sometimes I see what could just be new trends emerging or turning, and they wont show up in the pollster.com trendlines (yet) because they are too tentative.
And then on the other hand, you have the day-to-day news reports that uncritically zoom in on any one single opinion poll result, and act like it proves some new breakthrough. Like in one week in late November, one news story would conclude "Romney within 4 points of Giuliani", on the basis of a national ARG poll, while another concluded, "Romney down to 8%", based on a Fox News poll.
So, sometimes I want something of a middle way. Hence - new nimh graphics!
What I've been doing is taking the individual poll results (on both Iowa and NH), as registered on pollster.com, and calculating averages for bunches of five to eight polls at a time. So as the primary campaigns have heated up, the intervals have gone from two months to half a month.
The result is a bit more sensitive to short-term trends than the pollster.com trendlines -- but also more vulnerable to the impact of outliers and other statistical "noise".
Now in the image below I've compared the two graphs. On the right is my graph, as shown above but squeezed a bit; on the left the corresponding bit (1 June - 31 December) from the graph on pollster.com.
For those who dont know pollster.com; in its graph, individual (national) polls that come out are represented in dots: each dot represents the result of the candidate in question in one poll, placed on a timeline. On the basis of that data Prof. Franklin plots out the trendlines that you see. It's a regressive trendline (dont ask me) - it's not just a rolling average, but a trendline that is continually adjusted regressively as new polls come in. It's set up in such a way that it will not be impacted much by one or the other divergent individual poll result, it's only solid trends they want to pick up on.