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# What is a "Z Score"? Why is it used? What is the significance of a Z score when...

lonestargirl56 | Student, Professional | Valedictorian

Posted May 26, 2013 at 9:31 PM via web

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What is a "Z Score"? Why is it used? What is the significance of a Z score when analyzing something such as scores on a test?  Then respond to a classmate's postin

embizze | High School Teacher | (Level 1) Educator Emeritus

Posted May 27, 2013 at 1:17 AM (Answer #1)

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If a population is approximately "normal" (in the statistical sense) then you can convert the raw data of a sample into "z-scores" -- their analogous value on the standard normal scale where the mean=median=mode=0 and the standard deviation is 1.

The z-score helps to determine the percentage of the population that has a given characteristic (the z-score) or lower. (or higher depending on the application.)

If you have the scores on a test, and we assume that the scores will be approximately normally distributed, then you can convert each raw score (the actual test score) to its corresponding z-score. Then you can determine what percentage of the population will have a test score above or below the given test score from the value of the z-score in the standard normal table.

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