- Standard score
statistics, a standard score is a dimensionless quantity derived by subtracting the population meanfrom an individual raw score and then dividing the difference by the population standard deviation. This conversion process is called standardizing or normalizing.
Standard scores are also called z-values, "z"-scores, normal scores, and standardized variables.
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-factorused in the analysis of high-throughput screeningdata, but is sometimes confused with it.
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 rankmay be determined from the standard score and statistical tables.
tandardizing in mathematical statistics
mathematical statistics, a random variable"X" is standardized using the theoretical (population) mean and standard deviation:
where μ = E("X") is the
meanand σ = the standard deviationof the probability distributionof "X".
If the random variable under consideration is the
then the standardized version is
Richard J. Larsen and Morris L. Marx: "An Introduction to Mathematical Statistics and Its Applications, Third Edition," p. 282.
* [http://www.stats4students.com/Essentials/Standard-Score/Overview.php A Guide to Understanding & Calculating the Standard Score (Z-Score)]
* [http://www.acposb.on.ca/conversion.htm Z-Score to percentile conversion table] With a given Z-Score, calculate the value's percentile rank.
* [http://www.measuringusability.com/pcalcz.php Z-Score to percentile calculator] Converts Z-Scores into percentiles (1 & 2 Sided).
* [http://www.uark.edu/misc/lampinen/tutorials/normal.htm Normal Distribution & calculation of Z-Scores and percentile rank with Excel functions]
Z-Score Financial Analysis Tool
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