This confidence interval calculator estimates the margin of error/accuracy of a survey by considering its sample & population sizes and a given percentage of choosing specific choice. Please note that a safe confidence interval is set to be between 3 and 5 %.
What data you need to calculate the confidence interval
When assessing the level of accuracy of a survey, this confidence interval calculator takes account of the following data that should be provided:
- Confidence level that can take any value from the drop down list: 50%, 75%, 80%, 85%, 90%, 95%, 97%, 98%, 99%, 99.99%. Each confidence level from the ones provided above has its own Z score associated as detailed here:
- for confidence level 50% the Z Score is 0.67449;
- for confidence level 75% the Z Score is 1.15035;
- for confidence level 80% the Z Score is 1.28;
- for confidence level 85% the Z Score is 1.44;
- for confidence level 90% the Z Score is 1.645;
- for confidence level 95% the Z Score is 1.96;
- for confidence level 97% the Z Score is 2.17009;
- for confidence level 98% the Z Score is 2.326;
- for confidence level 99% the Z Score is 2.576;
- for confidence level 99.99% the Z Score is 3.29053.
- Sample size which is the number of people that will be interviewed.
- Population that can be left blank if population in infinite or can be provided as a finite value;
- Pick certain choice % refers to the percentage you expect people to pick up a certain choice from the possible answers.
Standard formulas used:
- Margin of error formula:
Za/2 * σ/√(n)
- State confidence interval equation:
x̅ ± Za/2 * σ/√(n)
Where:
x̅ = mean
Za/2 = confidence coefficient
a = confidence level
σ = standard deviation
n = sample size
13 Apr, 2015