Thursday, October 22, 2009

You are reading the polls and this chart incorrectly

TYWKIWDBI presents a chart aggregating polls from which seems to indicate that the Democrats and Republicans are losing voters who are becoming independent. Here is TYWKIWDBI's repost of the Pollster chart:

That conclusion can only be made if you actually ignore the points (besides the fact the the X axis labels are lost). Pollster has plunked a solid line which appears to be the average of the data from all of these polls. That's all well and good, but the points that the average is made from are all over the place. here I have replotted the polls and included the 90th and 10th percentile of the last 20 polls for each of the party affiliations. The heavy solid lines are the average of the last 20 polls, red for Republicans, blue for Democrats and green for Independents, the lighter solid lines are the 90th and 10th percentile of the last 20 polls. The points are the individual polls plotted on the ending date of the poll period. The data is all right under the chart at

Close examination (click on the chart above for a large version) of this very cluttered chart reveals that you could draw lines which trend up or down or stay the same for any any of the affiliations and not leave the area between the between the 90th and 10th percentile lines. Print it out and try it or replot it yourself.

The most you can confidently say about this chart is, "wow, there is a lot of scatter in these poll results." I don't think that you can state with confidence that there has been a statistically significant change in Democratic or Independent affiliation. I am not even sure there is a decrease in Republican affiliation. Shame on you for putting in those average lines, and shame on you TYWKIWDBI for reposting it with your faulty conclusions. Bad statistics, bad presentation (including my cluttered chart).

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