How accurate are state polls? And what could that mean for the US Presidential election in November?

Polls conducted in the final 21 days before the last five presidential general elections had a weighted average error of 4.0 points. 1 (We define error as the absolute difference between a poll’s margin between the top two candidates and the actual vote share margin. For example, if a poll gave the Republican candidate a lead of 3 percentage points but the Democrat won the election by 2 points, that poll had a 5-point error.) And even in 2016, when many people blamed the polls for not predicting President Trump’s victory, the polls within 21 days of the election performed decently well, with a weighted average error of 4.9 points.

But in the United States, the Electoral College picks the winner of presidential elections, which means state polls are what really matter. And state polls did have an off year in 2016 (although still pretty close to the long-term average). They had a weighted average error of 5.3 points, compared with 3.2 for national polls. Plus, that error systematically overestimated Democrats: State polls had a weighted average statistical bias (a metric that tells us which direction the error ran) of 3.5 points toward Hillary Clinton.

State polls are more error-prone than national polls

Weighted average error and statistical bias of national and state polls in the final 21 days before presidential general elections, among polls in FiveThirtyEight’s pollster ratings database

Weighted average error/Weighted average statistical bias

CYCLE	NATIONAL	STATE	NATIONAL	STATE
2000	3.8	4.6	R+3.1	R+2.2
2004	2.2	3.5	D+0.9	D+1.2
2008	2.2	3.8	0.0	D+1.2
2012	3.3	3.7	R+3.2	R+2.3
2016	3.2	5.3	D+2.4	D+3.5
All years	2.9	4.3	R+0.5	D+0.6

Averages are weighted by the square root of the number of polls that a particular pollster conducted in that particular cycle. Polls that are banned by FiveThirtyEight because we know or suspect they faked data are excluded from the analysis.

However, it is not unusual for state polls to be less accurate than national ones (although they did diverge a lot in 2016). Since 2000, state polls have a weighted average error of 4.3 points, while national polls have a weighted average error of 2.9 points. Some states have more accurate polls than others, though. Thanks to our pollster ratings data set, we can quantify which states’ presidential general election polls are the most and least accurate, which can help us better understand the state polls we’ll get later this year.

Swing state polls are usually pretty good.
Weighted average error and statistical bias of state polls in the final 21 days before presidential general elections, for states with at least 15 such polls, among polls in FiveThirtyEight’s pollster ratings database:

2000 ELECTION

STATE	NO. OF POLLS	WEIGHTED AVERAGE ERROR	WEIGHTED AVERAGE STATISTICAL BIAS
Michigan	21	4.1	R+3.6
Pennsylvania	21	3.8	R+2.3
New York	17	7.7	R+7.7
Florida	15	3.3	D+1.0

2012 ELECTION

STATE	NO. OF POLLS	WEIGHTED AVERAGE ERROR	WEIGHTED AVERAGE STATISTICAL BIAS
Ohio	44	1.8	R+0.2
Florida	32	2.4	R+1.9
Virginia	27	3.4	R+3.0
Colorado	21	4.0	R+3.9
Wisconsin	21	2.9	R+2.5
Iowa	19	3.8	R+3.6
New Hampshire	19	3.6	R+3.3

2016 ELECTION

STATE	NO. OF POLLS	WEIGHTED AVERAGE ERROR	WEIGHTED AVERAGE STATISTICAL BIAS
Florida	36	3.8	D+2.9
Pennsylvania	29	4.8	D+4.6
North Carolina	26	5.7	D+5.2
New Hampshire	21	4.8	D+3.2
Nevada	21	3.1	R+1.8
Michigan	20	4.7	D+4.4
Virginia	19	2.8	D+1.3
Ohio	18	5.8	D+5.8
Arizona	17	2.4	D+1.5
Colorado	16	2.7	R+1.3
Georgia	16	3.0	D+1.8

Averages are weighted by the square root of the number of polls that a particular pollster conducted in that particular cycle. Polls that are banned by FiveThirtyEight because we know or suspect they faked data are excluded from the analysis.