Question 55·Easy·Inference from Sample Statistics and Margin of Error
A random sample of 200 registered voters in a town found that would vote "Yes" on a ballot measure. The poll reports a margin of error of percentage points at the confidence level. The town has 18,000 registered voters. Based on this poll, which of the following is a reasonable estimate for the number of voters in the town who would vote "Yes"?
For SAT questions involving polls and margins of error, first convert the reported percentage and margin of error into an interval (e.g., 62% ± 4% becomes 58%–66%). Then connect that interval to the total population: either multiply the endpoints by the total to get a range of possible counts, or convert each answer choice to a percentage of the total and see which fall inside the interval. This comparison is usually faster and safer than trying to do more complicated statistics.
Hints
Use the margin of error
Start by combining the reported 62% with the ±4 percentage point margin of error. What interval of possible true percentages does that give you?
Relate percentages to actual numbers of voters
The town has 18,000 registered voters. How can you check what percentage of 18,000 each answer choice represents?
Compare each option to the plausible percentage range
Once you know the percentage that each answer choice corresponds to, compare those percentages to your interval from the margin of error. Which one falls inside that interval?
Desmos Guide
Find the plausible range of counts using the margin of error
In Desmos, type 0.58*18000 and 0.66*18000 on separate lines. These outputs give the lower and upper bounds on the number of voters consistent with the 58%–66% range.
Check which answer choice fits in that range
Now type each answer choice (9540, 10800, 12600, 8460) on separate lines. Look to see which of these four numbers lies between the two bounds you found in the previous step; that is the reasonable estimate.
Step-by-step Explanation
Use the margin of error to find the plausible percentage range
The poll result is 62% with a margin of error of ±4 percentage points.
So the likely range for the true percentage of voters who would vote "Yes" is:
That means the true proportion is likely between 58% and 66%.
Understand what a "reasonable estimate" means here
A reasonable estimate for the number of "Yes" voters should correspond to a percentage between 58% and 66% of the 18,000 voters.
So each answer choice should be viewed as:
- (answer choice) ÷ 18,000 = fraction of voters who would vote "Yes".
We will check which choice gives a percentage between 58% and 66%.
Convert each answer choice to a percentage of 18,000
Compute the fraction each answer choice represents of 18,000, then convert to a percent:
- A)
- B)
- C)
- D)
Now compare each of these percentages to the plausible range 58% to 66%.
Choose the percentage within the margin-of-error range
From the previous step:
- 53% (choice A) is below 58%.
- 60% (choice B) is between 58% and 66%.
- 70% (choice C) is above 66%.
- 47% (choice D) is below 58%.
Only 60% lies within the 58%–66% interval. Therefore, the reasonable estimate for the number of voters who would vote "Yes" is 10,800 (choice B).