Statistics
Find a standard deviation, turn a z, t or chi-square score into a p-value, and build a confidence interval, with the working shown for each.
Which calculator do I need?
- Describing a data set: the standard deviation calculator gives the mean, variance and standard deviation for a sample or a whole population.
- Testing a hypothesis: once you have a test statistic (z, t or chi-square), the p-value calculator gives its probability and compares it with your significance level.
- Estimating a value: the confidence interval calculator gives a range for a mean or a proportion at 90%, 95%, 99% or any level you choose.
Common questions
Should I use z or t?
Use z when the population standard deviation is known, or for a proportion. Use t when you estimate the standard deviation from your sample, with n − 1 degrees of freedom. With 30 degrees of freedom the 95% t value is 2.042, close to z = 1.960.
What does p < 0.05 mean?
It means that, if the null hypothesis were true, a result at least as extreme as yours would happen less than 5% of the time. At the 0.05 significance level you would reject the null hypothesis.