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October 24, 2012 - 7:03 pm
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RealClearPolitics publishes a continuously updated average of all major national presidential polls — as of this evening, their chart looks like this:


(Note that the RCP chart is updated frequently, so that by the time you read this, it may be slightly different.)

But whenever I check the RCP average, including today, I notice something odd: The larger the polling sample size, the more the poll favors Mitt Romney.

I’ve copied the RCP data and pasted it in here in a format that I can re-order. First, here’s how RCP organizes the chart, which they do chronologically, with the most recent poll at the top:

Poll Date Sample MoE Romney (R) Obama (D) Spread
RCP Average 10/15 – 10/23 47.8 47.2 Romney +0.6
Rasmussen Reports 10/21 – 10/23 1500 LV 3.0 50 46 Romney +4
ABC News/Wash Post 10/20 – 10/23 1394 LV 3.0 49 48 Romney +1
IBD/TIPP 10/18 – 10/23 938 LV 3.5 44 47 Obama +3
Gallup 10/17 – 10/23 2700 LV 2.0 50 47 Romney +3
Monmouth/SurveyUSA/Braun 10/18 – 10/21 1402 LV 2.6 48 45 Romney +3
CBS News 10/17 – 10/20 790 LV 4.0 46 48 Obama +2
NBC News/Wall St. Jrnl 10/17 – 10/20 816 LV 3.4 47 47 Tie
WashTimes/JZ Analytics* 10/18 – 10/20 800 LV 3.5 47 50 Obama +3
Politico/GWU/Battleground 10/15 – 10/18 1000 LV 3.1 49 47 Romney +2

But what if we simply re-ordered the polls not chronologically, but according to sample size, with the largest at the top? This is what it would look like:

Poll Date Sample MoE Romney (R) Obama (D) Spread
RCP Average 10/15 – 10/23 47.8 47.2 Romney +0.6
Gallup 10/17 – 10/23 2700 LV 2.0 50 47 Romney +3
Rasmussen Reports 10/21 – 10/23 1500 LV 3.0 50 46 Romney +4
Monmouth/SurveyUSA/Braun 10/18 – 10/21 1402 LV 2.6 48 45 Romney +3
ABC News/Wash Post 10/20 – 10/23 1394 LV 3.0 49 48 Romney +1
Politico/GWU/Battleground 10/15 – 10/18 1000 LV 3.1 49 47 Romney +2
IBD/TIPP 10/18 – 10/23 938 LV 3.5 44 47 Obama +3
NBC News/Wall St. Jrnl 10/17 – 10/20 816 LV 3.4 47 47 Tie
WashTimes/JZ Analytics* 10/18 – 10/20 800 LV 3.5 47 50 Obama +3
CBS News 10/17 – 10/20 790 LV 4.0 46 48 Obama +2

Notice the unmistakable trend?

All polls with 1000 or more respondents favor Romney; all polls with smaller than 1000 respondents favor Obama (or are tied).

Statisticians will tell you that the larger the sample size, the more reliable the poll. This fact is reflected in RCP’s “Margin of Error” (MoE) column, which shows a lower margin of error, and thus a greater level of reliability, for the large-sample pro-Romney polls. Each and every pro-Obama poll has a higher margin of error, and is thus less reliable.

These are the facts as they currently stand, and they’ve been true like this almost every day since soon after the first debate when Romney surged in popularity.

But Why?

The question we must now ask ourselves is why only the weak polls with low response rates favor Obama.

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