ref: data set (sample)
x: population mean
stddev: population standard deviation, defaults to the sample standard deviation
ZTEST calulates the probability of observing a sample mean as large as or larger than the mean of the given sample for samples drawn from a normal distribution with mean x and standard deviation stddev.
If ref contains less than two data items ZTEST returns #DIV/0! error.
This function is Excel compatible.
This function is OpenFormula compatible.