The most famous study of this phenomenon was included in an article written by Nick Panagakis, a member of the National Council on Public Polls, which article appeared in the February 27, 1989 edition of The Polling Report:
Most sophisticated political observers understand this dynamic, but there are still plenty of hacks in the media who don't. Rather than giving poll results their proper interpretation, too many reporters and news outlets continue to report them like football scores -- placing all the emphasis on the size of the "point-spread." This is particularly true among media outlets that do not subscribe to polls and do not have access to a professional analysis and interpretation of survey results. Too cheap to pay for the information, these news organizations rely on second-hand sources (condensed wire service accounts and campaign operatives) to acquire partial top-line numbers and report the "score." In a situation where an incumbent leads 48%-36% a week before an election, it is not uncommon for the challenger to eventually win 51%-49%. Still, the headline or lead story will scream "Governor Jones leads by 12-points in re-election bid."
How will undecideds vote on election day? Traditionally, there have been two schools of thought about how undecideds in trial heat match-ups will divide up at the ballot box. One is that they will break equally; the other, that they will split in proportion to poll respondents who stated a candidate preference.While a strong majority of the 21% undecided voters should go to Kennedy, one thing that militates against that movement is the fact Ballard does not have high negatives. It is very unusual that you see a poll in which an incumbent doesn't have high negatives, is said to be doing a good job, yet is well below 50%. So while most incumbents are topped out in their support by negatives, in this case Ballard actually does have some room to add undecided voters.
But our analysis of 155 polls reveals that, in races that include an incumbent, the traditional answers are wrong. Over 80% of the time, most or all of the undecideds voted for the challenger.
The 155 polls we collected and analyzed were the final polls conducted in each particular race; most were completed within two weeks of election day. They cover both general and primary elections, and Democratic and Republican incumbents. They are predominantly from statewide races, with a few U.S. House, mayoral and countywide contests thrown in. Most are from the 1986 and 1988 elections, although a few stretch back to the 1970s.
In 127 cases out of 155, most or all of the undecideds went for the challenger:
DISPOSITION OF UNDECIDED VOTERS
•Most to challenger....127
•Most to incumbent.....19
The fact that challengers received a majority of the undecided vote in 82% of the cases studied proves that undecideds do not split proportionally. If there were a tendency for them to split proportionally we would see most undecided voters moving to incumbents, since incumbents win most elections. Similarly, even accounting for sample error, it's clear from the chart above that undecideds do not split equally.
Mayor Greg Ballard
For poll users and reporters this phenomenon, which we call the Incumbent Rule, means: Incumbent races should not be characterized in terms of point spread. If a poll shows one candidate leading 50% to 40%, with 10% undecided, a 10-point spread will occur on election day only if undecideds split equally (i.e. a 55% to 45% outcome). Since most of the 10 points in the undecided category are likely to go to the challenger, polls are a lot closer than they look. 50% to 40% is likely to become 52% to 48%, on election day. If a poll is a mirror of public opinion, think of an incumbent poll as one in which objects are closer than they appear. (Emphasis added.()
An incumbent leading with less than 50% (against one challenger) is frequently in trouble; how much depends on how much less than 50%. A common pattern has been for incumbents ahead with 50% or less to end up losing. Final polls showing losing incumbents ahead are accurate. The important question is whether results are reported with an understanding of how undecideds decide. (My emphasis.)
The exceptions we found to the Incumbent Rule help support the theory on why this happens.
Many challengers who did not get a majority of undecideds were recent or current holders of an office equal to the one they were seeking. Voters were equally or more familiar with the challenger's past performance in a similar office, so the challenger assumed incumbent characteristics. Other exceptions include well-known challengers or short-term incumbents.
There is an interesting pattern in the polls where most undecideds voted for challengers. In 98 of the 127 cases (77%), the incumbents' final polls standing was plus or minus four percentage points from the actual election result. The most frequent result was two points gained by the incumbent over the final poll preferences -- 24 cases in all.
In 41 cases, or 32% of the 127, the incumbent ended with less than his stated poll percentage. This means that about one in four of all 155 polls actually overstated the incumbent's percentage.
Of the 127 challengers who gained more undecideds than did incumbents on election day, 78 gained 10 or more points over their stated poll percentage.
The overwhelming evidence is that an incumbent won't share the undecideds equally with the challenger. To suggest otherwise by emphasizing point spread or to say that an incumbent is ahead when his or her percentage is well under 50% leads to election day surprises.
This points to the validity of my view that, contrary to the claim Kennedy has been too negative, rather the challenger has been not nearly aggressive enough in going after Ballard and driving up his negatives. In particular, Kennedy should have hit Ballard on specific issues where he has taken very unpopular positions such as the Pacer multi-million dollar taxpayer giveaway and the 50 year ACS parking meter contract. The problem for Kennedy is that political strategy dictates that you define your opponent early, topping out his support, then you finish the campaign with a positive message to sweep up the undecideds. That is a lot of work for Kennedy to do with not much time left on the campaign clock.