Z score: Definition and Much More from Answers.com

Wikipedia: standard score

"Standardize" redirects here. For industrial and technical standards, see Standardization.

For Z-values in ecology, see Z-value.

In statistics, the standard score, also called the z-score or normal score, is a dimensionless quantity derived by subtracting the population mean from an individual raw score and then dividing the difference by the population standard deviation. This conversion process is called standardizing or normalizing.

The standard score indicates how many standard deviations an observation is above or below the mean. It allows comparison of observations from different normal distributions, which is done frequently in research.

The standard score is not the same as the z-factor used in the analysis of high-throughput screening data, but is sometimes confused with it.

Formula

The standard score is:
where:	x is a raw score to be standardized σ is the standard deviation of the population μ is the mean of the population.

The quantity z represents the distance between the raw score and the population mean in units of the standard deviation. z is negative when the raw score is below the mean, positive when above.

A key point is that calculating z requires the population mean and the population standard deviation, not the sample mean or sample deviation. It requires knowing the population parameters, not the statistics of a sample drawn from the population of interest. But knowing the true standard deviation of a population is often unrealistic except in cases such as standardized testing, where the entire population is measured. In cases where it is impossible to measure every member of a population, the standard deviation may be estimated using a random sample. For example, a population of people who smoke cigarettes is not fully measured.

When a population is normally distributed, the percentile rank may be determined from the standard score and statistical tables.

Standardizing in mathematical statistics

In mathematical statistics, a random variable X is standardized using the theoretical (population) mean and standard deviation:

where μ = E(X) is the mean and σ² = Var(X) the variance of the probability distribution of X.

If the random variable under consideration is the sample mean:

then the standardized version is

References

• Abdi, H. (2007). Z-scores. In N.J. Salkind (Ed.), Encyclopedia of Measurement and Statistics. Thousand Oaks, CA: Sage.

External links

• Z-Score to percentile conversion table With a given Z-Score, calculate the value's percentile rank.

See also

• Z-Score Financial Analysis Tool
• Z-test
• Z-factor
• moment (mathematics)
• central moment
• sampling distribution
• Student's t-test

Statistics
Descriptive statistics	Mean (Arithmetic, Geometric) - Median - Mode - Power - Variance - Standard deviation
Inferential statistics	Hypothesis testing - Significance - Null hypothesis/Alternate hypothesis - Error - Z-test - Student's t-test - Maximum likelihood - Standard score/Z score - P-value - Analysis of variance
Survival analysis	Survival function - Kaplan-Meier - Logrank test - Failure rate - Proportional hazards models
Probability distributions	Normal (bell curve) - Poisson - Bernoulli
Correlation	Confounding variable - Pearson product-moment correlation coefficient - Rank correlation (Spearman's rank correlation coefficient, Kendall tau rank correlation coefficient)
Regression analysis	Linear regression - Nonlinear regression - Logistic regression