FiveThirtyEight has three models that adjust and weight the polling average according to a number of factors – Polls-Plus, Polls-Only, and Now-Cast. Now-Cast has the least "smoothing" of the win percentage forecasts.
Here is an image of the most recent National Polling average produced by the Now-Cast model: https://i.sli.mg/eznBiZ.png
You can see that in this model Trump took a slim lead from July 25 through July 29th. So we would naturally expect many (although not all) polls from that period to show Trump in the lead.
When you scroll down you can see the most recent polls being given the highest weight by the model. If you click on "Show More Polls" you can see a list of the previous polls from earlier in the election season. Here is a screenshot of the Now-Cast for today, August 7th: http://sli.mg/LScz0n
Here is a picture of the polls from the period when Trump had a lead according to the 538 model (shown is any poll that included July 25th or July 29th): https://i.sli.mg/XOva19.jpg
Notice that not a single poll, after 538's adjustment, shows Trump with a lead. How is it possible that Trump had a lead for five days without ever once coming out ahead in 538's model? What happened to erase after-the-fact the lead that Trump had during that period?
I went to the Internet Archive's Wayback Machine to look for snapshots of the 538 National Polling average page, and the most recent snapshot they took (coincidentally) was on July 29th, the last day of Trump's lead. Here is the snapshot (you have to manually select "Now-Cast" in the sidebar to the left, because I can't link directly to it): https://web.archive.org/web/20160729151320/http://projects.fivethirtyeight.com/2016-election-forecast/national-polls/
What is immediately clear is that there are many polls showing Trump as the adjusted leader, as you would expect for Trump to show up as the leader in the chart produced by the model. How did this lead disappear between July 29th and today?
I decided to look at individual polls themselves to see what happened to the adjustment that 538 gives each poll. In their methodology page, Nate Pyrite explains the factors that go into their poll adjustments. The most important is the "house effects" adjustment, that corrects for the bias of each individual polling firm, some of which are known to produce results favoring Republicans and others favoring Democrats to different degrees.
So what happened to the adjustments of polls from the period of Trump's lead? Below is a table showing what happened to each individual poll between July 29th and now. Adjustment is shown in the table as negative when it perceives the poll as biased toward Clinton and corrects toward Trump, and positive when it perceives the poll as biased toward Trump and corrects toward Clinton.
Poll |
July 29th Adj |
Today's Adj |
Change |
Jul. 22-26 Ipsos |
-2 |
+3 |
+5 |
Jul. 21-22 Gravis Marketing |
0 |
+6 |
+6 |
Jul. 22-24 CBS News/New York Times |
0 |
+5 |
+5 |
Jul. 22-28 USC Dornsife/LA Times |
+3 |
+9 |
+6 |
Jul. 22-24 Morning Consult |
-1 |
+5 |
+6 |
Jul. 23-24 YouGov |
+1 |
+6 |
+5 |
Jul. 22-24 CNN/Opinion Research Corp. |
-1 |
+5 |
+6 |
Jul. 26-27 Rasmussen/Pulse Opinion Research |
+1 |
+5 |
+4 |
Jul. 18-24 SurveyMonkey |
-2 |
+4 |
+6 |
Jul. 22 RABA Research |
-2 |
+5 |
+7 |
Jul. 14-16 Monmouth University |
-3 |
+5 |
+8 |
Jul. 12-16 RKM Research and Communications, Inc. |
-2 |
+6 |
+8 |
Jul. 21-24 University of Delaware |
-1 |
+5 |
+6 |
Jul. 9-13 NBC News/Wall Street Journal |
-4 |
+3 |
+7 |
Jul. 13-18 Greenberg Quinlan Rosner |
-4 |
+3 |
+7 |
Jul. 21-22 Echelon Insights |
-1 |
+6 |
+7 |
Jul. 5-9 Marist College |
-5 |
+3 |
+8 |
Jul. 17-20 American Research Group |
-1 |
+5 |
+6 |
Jul. 11-14 ABC News/Washington Post |
-1 |
+6 |
+7 |
Jul. 12-15 icitizen |
-3 |
+4 |
+7 |
Jul. 7-11 GfK Group |
-3 |
+4 |
+7 |
Jul. 8-12 CBS News/New York Times |
-3 |
+5 |
+8 |
Jul. 13-16 CNN/Opinion Research Corp. |
-3 |
+4 |
+7 |
Jun. 21-27 Quinnipiac University |
-3 |
+5 |
+8 |
Jun. 26-28 Fox News |
-4 |
+4 |
+8 |
Jun. 26-29 Suffolk University |
-5 |
+4 |
+9 |
Jun. 15-26 Pew Research Center |
-6 |
+2 |
+8 |
Jul. 15-17 YouGov |
-1 |
+6 |
+7 |
Jun. 27-28 Public Policy Polling |
-4 |
+4 |
+8 |
Jun. 24-29 IBD/TIPP |
-5 |
+3 |
+8 |
Jun. 28-29 Zogby Interactive/JZ Analytics |
-7 |
+1 |
+8 |
Jun. 20-23 ABC News/Washington Post |
-4 |
+4 |
+8 |
Jun. 8 SurveyUSA |
-5 |
+4 |
+9 |
Jun. 19-24 Atlantic Media/Heartland Monitor |
-6 |
+2 |
+8 |
Jun. 15-19 Monmouth University |
-6 |
+3 |
+9 |
Jul. 21-25 Ipsos |
-2 |
+3 |
+5 |
Jul. 18-19 Rasmussen/Pulse Opinion Research |
0 |
+7 |
+7 |
Jun. 10-13 Selzer & Company |
-8 |
0 |
+8 |
Jun. 19-23 NBC News/Wall Street Journal |
-7 |
+2 |
+9 |
Average |
|
|
+7 |
I could continue, but you get the point. Here is a spreadsheet comparing every single poll from July 29th to its counterpart today, August 7th. The average re-adjustment in favor of Clinton was 7.9 points.
In the Now-Cast today, Nate Pyrite has Clinton leading Trump by 7.5 points (44.5 to 37). But this post-DNC bounce may have been entirely manufactured by re-weighting every single polling firm by 4-9 points in Clinton's favor. Is it remotely plausible that the house effects of every single polling firm were discovered since July 29th to be heavily biased towards Trump, necessitating radical across-the-board re-adjustment? And that this discovery just so happened to coincide with the end of the DNC? Not even remotely plausible.
Given that Hillary's lead today is reported to be 7.5 points, but the above table shows an average shift of 7 points in Hillary's favor, it is plausible that Hillary's lead today would be only 0.5 points if the weighting from July 29th were still being used.
Nate Pyrite has clearly and ham-handedly cooked the books in order to produce a post-DNC bounce for Hillary Clinton, sacrificing what little integrity and honesty he might still have had before this.
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