PPP’s baffling discard process

by Andrew Gelman on September 10, 2013 · 5 comments

in Methodology,Public opinion

B. J. Martino writes:

Earlier this summer when I went on a bit of a rant about PPP and their process of discarding interviews, rather than simply weighting data. Mark Blumenthal mentioned your response to the discusion in one of his posts, where you said you were a bit “baffled” by it.

While they claim to engage in the discard process as a kind of retroactive quota to account for more older, white women in their sample, it was the discards among the non-”older white women” that made me curious. That is, any respondent who was not meeting all criteria of being age 46+, white and female.

I downloaded the data from all their 2012 surveys for Daily Kos/SEIU, and compared the sample of non-”older white women” within the unweighted released data as well as the discarded data.

At least from the first six surveys I have looked at, there appears to be a consistent difference in the partisan composition of the released data and the discarded data for this group. In every case, the released data for this group was net Democratic in Party ID (Unw D-R), and the discarded data was net Republican (Dis D-R).

Party ID in PPP Polls for Daily Kos/SEIU- Non-”older white women”
(raw unweighted data and discarded data)




























































 

Unw Sample



Discarded Sample



Unw D-R



Dis D-R



Unw- Disc



25-Oct



968



335



6.5%



-1.5%



8.0%



12-Oct



1100



302



6.5%



-8.6%



15.1%



4-Oct



922



356



5.1%



-19.7%



24.8%



27-Sep



763



277



10.2%



-8.6%



18.8%



20-Sep



829



236



12.0%



-18.2%



30.2%



13-Sep



699



267



8.7%



-7.9%



16.6%



What this suggests to me is that the discard process is both a way to apply a retroactive quota to older white women, but also a way to fix the partisanship from another group (assuming this is primarily younger voters). My thought is that they are getting too Republican a sample in this group because they never dial cell phones.

I found it interesting that despite hanging their hat on being the most accurate of 2012, they announced today that they would be working to find a way to include cell phones in the future.


Martino continues:
As I told Mark, I’m not really interested into getting into a shouting match with PPP. It has always just kind of dumbfounded me how they work. The fact that Daily Kos/SEIU published all the raw data from PPP’s 2012 polling at least gave me some opportunity to figure it out.

I guess the troubling part is how they have repeatedly stood by the statement that they do not weight for Party ID, when this discard process would seem to indicate a de facto weight on Party ID for at least a portion of the sample. What they say is strictly true, but the effect is the same. Seems to be arguing semantics.

I also took a look at the Presidential ballot for this same group of non-“older white women.” Same effect, perhaps even a bit more pronounced.

Presidential Ballot in PPP Polls for Daily Kos/SEIU- Non-older white women
(raw unweighted data and discarded data)




























































 

Unw Sample



Discarded Sample



Unw O-R



Dis O-R



Unw- Disc



25-Oct



968



335



4.2%



-11.6%



15.8%



12-Oct



1100



302



-0.4%



-19.2%



19.6%



4-Oct



922



356



-0.4%



-30.4%



30.8%



27-Sep



763



277



10.3%



-11.2%



21.5%



20-Sep



829



236



9.7%



-22.1%



31.8%



13-Sep



699



267



6.3%



-15.0%



21.3%



 

I don’t really have anything to add here; it’s just an interesting story. I remain amazed that anyone would think it’s a good idea to throw away survey interviews that have already been conducted.

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