Interpreting Gay Marriage Poll Results: Do the Bumps and Wiggles Mean Anything?

This is a second guest post by Ryan Enos and Lynn Vavreck.  Their first one on this subject is here.


In 1960, Richard Neustadt wrote Presidential Power, in which he told students of politics that presidential power was the “power to persuade.”  Neustadt wasn’t so much talking about shifting public opinion, but Sam Kernell introduced that wrinkle in 1986.   On May 9, President Obama announced that his opinion on gay marriage had changed – and ever since, analysts of public opinion have been looking for hints of presidential power.  Can the president move public opinion?  Did this president move opinion on this issue?

The wrinkle to exploit is that the president is black – and support for gay marriage among African Americans Democrats is much lower than it is among white democrats.  In fact, black democrats look more like white Romney supporters than white Obama supporters, roughly (see graphic below).  This unusual pattern presents an opportunity to witness a president leading opinion on a topic.  Will support for gay marriage among black democrats increase now that the president has endorsed it?

Our answer to this question first appeared here.  John Sides at The Monkey Cage summarized the ensuing debate here.  And Ezra Klein similarly, here.  All of us come down on the side of caution – if Obama is leading opinion-change on this topic, it is very hard to see it in the data.   We have been asking a nationally representative sample of voters about support for gay marriage since February 4th of this year.  Take a look at support (of any intensity) for gay marriage over time among three groups:  black Obama supporters, white Obama supporters, and white Romney supporters.

These data are from weekly YouGov polls of a nationally representative sample of 1000 Americans, of which approximately 11% are black.  We’ve plotted average levels of support for gay marriage over 16 weeks, which means we have roughly 16,000 observations of which 1,850 are black and 11,679 are white.

The first thing to notice in the figure is the increased level of variation among black Obama supporters relative to the other groups.  This is due to the size of that subsample (only 1,850 blacks compared to 11, 679 whites).  There are more than six times as many whites in the sample as blacks, thus the estimates of black opinion will be measured with more imprecision relative to white opinion.

The second obvious pattern in these data is the constant motion of each line – a little increase here, a little decrease there.  Some of our political science colleagues call these kinds of patterns “bumps and wiggles.”  Interpreting these bumps and wiggles, as any scholar of campaigns will tell you, is risky  – especially after the fact.  The danger is two-fold:  first, there is the undeniable urge to look for the movements and then find some cause that explains them.  This is essentially identifying shifts in public opinion and looking for events that might have caused them, not terribly satisfying as science goes.  A better approach would be to stipulate beforehand a set of things that might drive movement and then place those markers on the graph looking to see if they coincide with movement in opinion.

The second danger is in wanting to interpret the movement after a focal moment – like Obama’s announcement – as important, even if it resembles the average weekly fluctuations that came before it.  Or worse, even it the movement after the important event is smaller than the average weekly fluctuations in the series—presumably most of which were not driven by similar moments!

For example, the average weekly change in support for gay marriage among African Americans who support Obama is 10 points.  The range of these changes goes from a whopping 21.5 points between March 24 and March 31to a low of -1.25 points the next week (April 7).  There are also very large changes in support for gay marriage among this group for the weeks ending April 21 (19 points), March 24 (-19 points), February 19(14 points), and March 4and April 14 (-15 and -14 points respectively).

The week after Obama’s announcement, support for gay marriage among black Obama supporters went down by 2.5 points.  The following week if went up by 6.5 points.  These changes are small in comparison to the changes between each of the other pairs of weeks.  They are in fact smaller than the average weekly change since February 4.  And importantly, we do not expect opinion on gay marriage to be fluctuating in this way week to week in response to political events.  These changes are random noise; they reflect the uncertainty associated with each estimate of support.

Another way to see the danger in interpreting poll movements in isolation is to look at the changes from February 20-26.  Support among black Obama supporters went up by 6.7-points, exactly the same amount of change the data show after Obama’s announcement from May 12-19.   But on February 26 support jumped up to 54 percent – the highest in the series.  On May 19, it jumped to only 47 percent.  If we want to interpret the post-Obama 6-point surge as meaningful, we better have a similar story for February 26– and while we are at it, for all the other weeks (8 of the remaining 14!) between which the changes in support were greater than 6 points.

February 26, it turns out, did end an important week of news about gay marriage: the Maryland legislature, on May 23, approved Gov. Martin O’Malley’s bill to legalize same-sex marriage.  The vote in the Senate was close, and it got a substantial amount of coverage in newspapers across the country.  So, if we engage in a little “bump and wiggle” analysis of these changes, we can report that Obama is leading black opinion on the topic about as much as the Maryland Governor and legislature – although the latter managed to actually push support for gay marriage among blacks up over the 50 percent mark, which Obama has yet to do.

Separating out strong supporters from weak supporters of gay marriage does not change these patterns, as the figure below makes clear.


The dashed lines are the average support within each group before Obama’s announcement.  Orange bars include supporters of any intensity and purple bars are only people who “strongly” support the policy.

The same kinds of patterns can be seen among white voters as well – both those voting for Obama and Romney.  Among white Obama voters’ support for gay marriage is at it highest on May 19 after the announcement, but another week in which it hit this zenith is February 26 – after the Maryland law was passed.   But mainly, among white voters you can see the ebb and flow of the noise in these polls.  Support goes up and then it goes back down, over and over.  The magnitude of the swings are not as big as they are among blacks because of the increased sample size for white voters relative to black voters, but the warning is the same:  beware of interpreting the bumps and wiggles.

3 Responses to Interpreting Gay Marriage Poll Results: Do the Bumps and Wiggles Mean Anything?

  1. Michael Tesler June 1, 2012 at 9:41 am #

    Great post on the dangers of bump and wiggleology! But there seems to be something more systematic going on with black support for gay marriage than just irresponsible analysts capitalizing on large swings arising from small subsamples in opinion polls. Even with such small subsamples, the Washington Post Poll reports a statistically significant increase in black support from before to after Obama’s announcement. PPP’s Maryland poll included an oversample of 398 African-Americans, so its 16 point increase in black support for marriage equality following Obama’s announcement is highly significant. PPP’s North Carolina poll doesn’t report its black sample size, but the 11 point drop in opposition to marriage equality following Obama’s announcement also appears to be significant given the sates’ sizable black population. Finally, the 8 point increase in black support for gay marriage found in PPP’s Pennsylvania Poll may not be significant; yet taken together, these four polls suggest that the Obama effect on black public opinion is a real phenomenon.

  2. Andy Rudalevige June 1, 2012 at 9:04 pm #

    Great post. I would quibble only with the idea that no one had talked about the presidential role in public opinion until _Going Public_ (which people should certainly read). In fact one of the core resources Neustadt identifies as fueling presidential persuasion is “public prestige.” His take was that presidents could build their approval/prestige by educating voters about key issues affecting their daily lives, linking his position to solutions to those issues. That logic might lead us not to expect very much impact from a single statement simply taking a position.

  3. Manju June 1, 2012 at 10:20 pm #

    The bumps and wiggles are clearly leading us toward gay marriage…after which they will most definitely stop.