Posted by George on Wednesday, May 5, 2010 at 7:02pm.
Use a proportional confidence interval formula.
CI90 = p + or - (z-value)(√ pq/n)
Find z-value using a z-table representing 90%. p = 643/1037 (convert to a decimal). q = 1 - p. n = 1037 (sample size). Plug values into the formula and calculate the interval.
Formula to find sample size:
n = [(z-value)^2 * p * q]/E^2
... where n = sample size, z-value is the same as above, p and q are the same as above, ^2 means squared, * means to multiply, and E = .02 (for 2%).
Plug values into the formula and calculate n.
I hope this will help get you started.
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