Over the last day or two, you might have seen the referendum data from May/June published by the British Election Study. It shows a relatively narrow (albeit hardly untypical) No lead of 51% to 39%, which is down from a 52% to 37% split in February/March. But what I didn't realise initially was that the fieldwork was conducted by YouGov, whose panel is notorious for producing higher No leads than the average (both in polls for YouGov themselves, and in Progressive Partnership polls for which YouGov conducted the fieldwork). There has been no official YouGov poll so far in this campaign that has shown a No lead of as low as 12% (the lowest was 14%), and nor has there been one that has shown a Yes vote of as high as 39% (the highest was 37%). Yet the unusually narrow gap in the BES data has been found in spite of the sample size being five times bigger than for a standard YouGov poll.
Overnight, I was sent the following commentary on the survey results by someone who said "no need to mention me" - I'm not sure whether that was a specific request for anonymity or not, but to be on the safe side I'll assume that it was! Incidentally, I've changed the Wave 1 Yes figure below from 38.6% to 36.6%, which I'm pretty sure is the correct number.
I am a YES supporter and canvass in Edinburgh. I am not an expert on opinion poll methodology but analyse statistical data from scientific experiments so have some familiarity with issues involved in estimation.
I thought I would pass on some information about the "British Election Study" that was released today. I was at the academic launch meeting in Edinburgh today.
Some detail on methodology
The data was collected **online** by **YouGov** on behalf of BES. The Wave 1 (i.e. 20 Feb to 09 March) sample was N=5896 and Wave 2 (i.e. 22 May to 18 June) was N=6182. The percentages for Wave 1 was 52.0% NO, 36.6% YES and 11.4% undecided. Wave 2 was 51.0% NO, 38.8% YES and undecideds 10.2%. I copied these figures from the powerpoint presentation of Prof Ed Fieldhouse.
Although it can be classified, with regards the IndyRef, as a YouGov poll, many other questions were added (over 200, on a wide range of topics). These 6000 Scots voters were part of a larger sample that had 20,000 English and 3,000 Welsh. The Scottish referendum question was not asked upfront therefore. Also, as part of a GB poll, it did not include voters who were 16 and 17.
Prof Curtice chaired the meeting and commented that, for a YouGov poll, it was not as NO-friendly as expected.
I have some other hand-written notes but I hope this helps to put it in context.
NOTE for OPINION POLL GEEKS (like me :-)
The BES polling data was collected by YOUGOV on behalf of BES.
This poll should thus be compared to recent YouGov IndyRef polls. YouGov is one of the 6 polling companies charting the progress of the referendum, and tends to produce high leads for the NO camp. Polls have shown NO leads ranging from 3% (Panelbase) to 19% (YouGov) recently.
These results above show NO at 51.0%,YES at 38.8% and UNDECIDED at 10.2% (data collected 22 May to 18 June, sample size 6162) at the later date and NO 52.0%, YES 36.6%, UNDECIDED 11.4% (data collected 20 Feb to 09 March), sample size N-5896) at the earlier date.
Note also that 16 and 17 year-olds were not included.
COMPARISON with YouGov polls
These N=6000 polls can be compared with YouGov polls with N=1200 samples. However, note that these larger polls ask other questions (over 200 questions) and the Scottish IndyRef question is not asked upfront. Note also that 16-17 year olds were not included in the N=6000 polls.
N=6000 polls and N-1250 YouGov polls
These two snapshots in Feb/March and May/June have NO leads of 15.4% and 12.2% respectively, based on samples of 6000 voters each.
Standard Indyref polls with sample sizes of about 1200 in approximately the same period showed a NO lead of 18% (24-28 March, N=1257) and a NO lead of 19% (25-29 June, N=1206).
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One point I should add for clarity is that YouGov never interview 16 and 17 year olds, even for dedicated referendum polls - they're the only pollster that still fails to do so.