In 95% confidence interval for the population proportion of successes
exists CI = 0.625 +/- 1.96 × sqrt(0.625 × 0.375/80).
In 95% confidence interval for the population proportion of failures
exists CI = 0.375 +- 1.96 × sqrt(0.375 × 0.625/80).
The mean of your estimate plus and minus the range of that estimate constitutes a confidence interval. Within a specific level of confidence, this is the range of values you anticipate your estimate to fall within if you repeat the test. In statistics, confidence is another word for probability.
You have a 5% probability of being incorrect with a 95% confidence interval. You have a 10% probability of being incorrect with a 90% confidence interval. A 95% confidence interval is narrower than a 99% confidence interval (for example, plus or minus 4.5 percent instead of 3.5 percent).
a) Sample proportion of successes (phat) = 50/80 = 0.625
z-critical for 95% CI is 1.96
CI = 0.625 +/- 1.96 × sqrt(0.625 × 0.375/80)
b) sample proportion of failures (phat) = 30/80 = 0.375
CI = 0.375 +- 1.96 × sqrt(0.375 × 0.625/80)
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