ArpitGarg's Weblog

A quick look at Real Clear Politics (RCP) poll page will show you that poll numbers between Trump and Clinton are hugely different on various opinion polls. They vary from +12 in favor of Clinton to +7 in favor of Trump. RCP uses few latest polls and takes their average to give a number. But the question is why so much difference? There are a few reasons as per what I understand.

1. Who are People polled: Most important is the demographic of the respondents. Votes can be broadly divided into registered Democrats, registered Republicans and Independents. Registered Democrats are more likely to vote for Clinton while Registered Republicans are more likely to vote for Trump. If majority of voters polled are Democrats then results will be skewed in favor of Democrat candidate and same goes for Republicans. Hence pollsters divide the voters polled evenly between Democrat and Republican to get a clearer picture. How much ratio a pollster keeps, decides the poll number. Some use actual/exit-poll data from last election to decide on this ratio. Some predict the ratio on basis of the current mood of the public. For example in 2008 Democrat turnout was huge due to Obama wave which was different than in 2004. Different polls keep different ratio of Democrats/Republicans and hence different number.
2. LV/RV: Likely Voters v/s Registered Voters. Some pollsters poll RV, some poll LV. Registered voters are the ones registered to vote. LV’s are the ones that are likely to vote in the election out of the Registered Voters. This LV formula differs among pollsters, causing the differences. Likely can again be based on party affiliation, age, ethnic background among other things.
3. Political Leaning of Pollster: This is one if the major factors believe it or not. Pollsters themselves have their own political leanings. They tend to be biased. More so in a polarized environment. They think they can sway the public opinion especially in the early polls.
4. Quid pro quo: In today’s world reputation is the last thing that gets you money. Some quid pro quo with one side or the other, goes a long way in the practical world and hence the discrepancy.