Yi is running for class president. to try to predict how people will vote, she wants to take a poll with a 90% confidence level (z*-score of 1.645) and an estimated margin of error of 4%. what is the approximate minimum sample size for yi’s initial poll?

90The information that you'd need in determining sample size when the standard deviation is unknown is:

z-value for the given level of confidence ⇒ The level of confidence is 90% and the z-value by convention is 1.645

The margin error (E) = 4%

The proportion of the population that you'd expect to vote for Yi. If this information isn't provided, use 50%. We call this value, p, the probability of success.

The probability of not success is q = 1 - 0.5 = 0.5

STEP 1:
Half the margin error = 0.04 ÷ 2 = 0.02

STEP 2:
Multiply the probability of success by the probability of fail = 0.5 × 0.5 = 0.25

STEP 3: 
Divide z-score by half of E = 1.645 ÷ 0.02 = 82.25
Then square the answer = 82.25² = 6765 rounded to the nearest integer)

STEP 4: 
Multiply the answer from STEP 2 by the answer from STEP 4
0.25 × 6765 = 1691 (rounded to the nearest integer)

Minimum Sample Size is 1691

In this problem it decreased 20%