The Quick Count and Election Observation

CHAPTER FIVE: STATISTICAL PRINCIPLES AND QUICK COUNTS
78 The margin of error increases as polling station size increases. The overall effect
of polling station size on margin of error, however, decreases as both rise.
Figure 5-7 illustrates this point.
FIGURE 5-7:
POLLING STATION SIZE
AND MARGIN OF ERROR
12
7
99% confidence level
Margin of Error
6
5
4
3
2
95% confidence level
1
0
150 200 250 300 350 400 450 500
Size of Polling Station (Voters/Station)
When elections are
very close, quick count
analysts must also be
concerned with the
level of voter turnout.
Correcting for Turnout
When elections are very close, quick count analysts must also be concerned
with the level of voter turnout. Even if observers have been successful at retrieving
data from each of the 1,020 polling stations in the theoretical sample, low
voter turnout will mean that there will be fewer votes included in the sample
than if turnout had been high. The original calculation was based on the expectation
of some 160 votes per polling station. If turnout has been at 70 percent,
however, there would only be 112 votes at each polling station. If that pattern
is repeated across the 1,020 polling stations, then the count would include
only 114,240 votes, some 50,000 shy of the desired 163,185 needed to achieve
a margin of error of 0.3 percent and a confidence level of 99 percent.
FIGURE 5-8:
TURNOUT AND MARGIN OF ERROR
TURNOUT
# VOTERS,
VOTES
MARGIN OF ERROR
(confidence levels)
95% 99%
Desired sample
(turnout = 100%)
163,185 ±0.24 ±0.31
90% 146,867 ±0.25 ±0.33
80% 130,548 ±0.27 ±0.36
70% 114,230 ±0.29 ±0.38
60% 97,911 ±0.31 ±0.41
50% 81,593 ±0.38 ±0.49