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What is Z-Value?

Z-value, also known as a Z-score, is a statistical measure that quantifies the distance (in standard deviations) a data point is from the mean of a dataset. It standardizes data to a standard normal distribution with a mean of 0 and a standard deviation of 1. This data denotes the likelihood of z-values falling within a segment of the standard normal distribution. Employ a z-value table to discover probabilities associated with intervals of z-scores and for determining p-values in z-tests.

Illustration indicating the correlation between z-scores and the left-side area. The z-table is split into negative and positive z-scores. Negative z-scores lie beneath the mean, whereas positive z-scores reside above it. The z-score is described by row and column headers, while the table cells exhibit the corresponding area.

The formula to calculate the Z-value of an individual data point “x” in a dataset with mean “μ” and standard deviation “σ” is:

Z=xμ / σ


  • x is the individual data point.
  • μ is the mean of the dataset.
  • σ is the standard deviation of the dataset.
  • Z is the Z-score of the data point.

The Z-score tells you how many standard deviations a data point is above or below the mean. A positive Z-score indicates that the data point is above the mean, while a negative Z-score indicates that it is below the mean. A Z-score of 0 means the data point is exactly at the mean.

Uses of Z-Values

  1. Normalization and Standardization: Z-values are used to standardize data in a table, making it easier to compare different datasets with different means and standard deviations.
  2. Outlier Detection: Z-scores help in identifying outliers by highlighting data points that significantly deviate from the mean.
  3. Hypothesis Testing: Z-tests utilize Z-values to determine the significance of the difference between a sample mean and a population mean.

Z-values tables are widely used in various fields such as finance, economics, biology, and psychology for analyzing and interpreting data in a standardized manner, enabling comparisons and statistical inferences across different datasets.

How to Use a Z-Value Table

  1. Finding Probabilities: Use Z-Value to find the probability that a standard normal random variable is less than a particular Z-score “Z”, you look up the Z-score in the table and read the corresponding probability. This gives the area under the standard normal curve to the left of that Z-score.
  2. Finding Z-scores: Conversely, if you’re given a probability and want to find the Z-score corresponding to that probability, you locate the probability in the table and read off the Z-score associated with that probability.

Z-value tables are useful for various statistical calculations and hypothesis testing. They help in determining the probabilities associated with certain Z-scores or standard normal random variables without having to perform complex calculations. However, with the advancement of technology and statistical software, many calculations involving Z-scores and probabilities are now performed using computational tools rather than manual lookups in tables.