Z-score Explorer
Where does one observation fall relative to the rest?
Your observation on a standard normal curve
Drag the orange marker to change the observation ↔Probabilities and percentiles above assume a normal model. A z-score can be calculated for nonnormal data, but the normal curve may not describe their probabilities.
Your actual sample
The empirical proportion counts your actual observations. It may differ from the normal-model percentile. Moving the orange marker does not change the loaded data.
Z-score versus t-test
A z-score: How far is one value from the mean, in standard deviations?
A t-test: How large is an estimated mean difference relative to its standard error?
Which standard deviation should I use?
If you know the population mean and standard deviation, enter those. If you load a sample, the app uses its mean and sample standard deviation (with n − 1 in the variance calculation). These are descriptive standardized scores; their normal-model areas are approximations, not t-test p-values.