Z-score is a widely used standardized score in statistics. It is used when the population parameters (mean and standard deviation) are known. When the population parameters are unknown and only those of samples are known the Student’s t-statistic is used. Z-score derives its name from the normal distribution which is also known as the “z distribution” in statistics. A positive z-value indicates a standard score which is above the mean, while a negative corresponds to a below the mean score.
It is a very useful statistic for calculating the probability of a value occurring within a normal distribution. Z-score also allows a comparison of two scores which come from different normal distributions. This is achieved by converting raw to standardized scores.