RealClearPolitics’ Misleading “Poll of Polls”

For my latest analysis of trends in RCP’s polls and several others, see “Election 2016.”

REVISED AND UPDATED 10/23/16

A lot of commentators cite the “poll of polls” at RealClearPolitics.com. You know the one I mean; it looks like this:

rcp-poll_1
rcp-poll_2

The graph is followed by a long list of historical polling results, on which the graph is based. It all looks authoritative. But it’s misleading.

Take the values for September 22, 2016, which show a spread of 2.1 points in favor of Clinton.  However, the values for September 22 represent polling that was done between September 8 and September 21. That’s quite a lag and it badly distorts what’s really happening in the Trump-Clinton race.

So I reconstructed the “poll of polls,” as follows:

  • Assigned a date to each poll that coincides with the central date of the polling period it represents.
  • Computed, for each poll, the spread in favor of (or against) Clinton.
  • Arranged the polls in chronological order, according to central date.
  • Averaged the spreads for polls having the same central date.

Because only one or two polls are assigned to many dates, I added a trendline to emphasize the pattern that emerges from the many polls included in RCP’s “poll of polls.” Here’s a graph of the result, for polls conducted since August 1:

clintons-lead-deficit-in-rcp-polls

That’s a much more realistic depiction.