# Archive for the ‘Glossary’ Category

*Key Terms and Defiinitions (c.2008, William T. Beverly, Ph.d.)*

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Posted by nepeht on November 24, 2008

http://en.wikipedia.org/wiki/Percentile.

A percentile is the value of a variable below which a certain percent of observations fall. So the 20th percentile is the value (or score) below which 20 percent of the observations may be found. The term percentile and the related term percentile rank are often used in descriptive statistics as well as in the reporting of scores from norm-referenced tests.

The 25th percentile is also known as the first quartile(Q1); the 50th percentile as the median or second quartile(Q2); the 75th percentile as the third quartile (Q3).

http://en.wikipedia.org/wiki/Percentile_rank.

The percentile rank of a score is the percentage of scores in its frequency distribution which are lower. For example, a test score which is greater than 85% of the scores of people taking the test is said to be at the 85th percentile. Percentile ranks are commonly used to clarify the interpretation of scores on standardized tests. For the test theory, the percentile rank of a raw score is interpreted as the percentages of examinees in the norm group who scored below the score of interest.[1]

Percentile ranks (PRs or “percentiles”) are normally distributed and bell-shaped while normal curve equivalents (NCEs) are uniform and rectangular in shape. Percentile ranks are not on an equal-interval scale; that is, the difference between any two scores is not the same between any other two scores. For example, 50 – 25 = 25 is not the same distance as 60 – 35 = 25 because of the bell-curve shape of the distribution. Some percentile ranks are closer to some than others. Percentile rank 30 is closer on the bell curve to 40 than it is to 20.

From Wikipedia

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Posted by nepeht on November 22, 2008

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Posted by nepeht on November 22, 2008

http://www.graphpad.com/articles/pvalue.htm.

A discussion of the meanings of P Value, Sample and Population.

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Posted by nepeht on November 22, 2008

http://en.wikipedia.org/wiki/P-value.

In statistical hypothesis testing, the p-value is the probability of obtaining a result at least as extreme as the one that was actually observed, given that the null hypothesis is true. The fact that p-values are based on this assumption is crucial to their correct interpretation.

More technically, a p-value of an experiment is a random variable defined over the sample space of the experiment such that its distribution under the null hypothesis is uniform on the interval [0,1]. Many p-values can be defined for the same experiment.

From Wikipedia.

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Posted by nepeht on November 21, 2008

http://www.chem.utoronto.ca/coursenotes/analsci/StatsTutorial/MeasMeanVar.html.

Helps explain the relationship between the Variance and Standard Deviation. From the Chemistry Department at the University of Toronto.

Posted in Glossary, Notes | Tagged: Mean, Standard Deviation, Variance | Leave a Comment »

Posted by nepeht on November 19, 2008

http://en.wikipedia.org/wiki/Standard_score.

In statistics, a standard 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.

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-factor used in the analysis of high-throughput screening data, but is sometimes confused with it.

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Posted by nepeht on November 15, 2008

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Posted by nepeht on November 9, 2008

Normal is defined as, “something regarded as a normative example” or “conforming with or constituting a norm or standard or level or type or social norm” by http://freedictionary.org/.

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Posted by nepeht on August 4, 2008

Standardization is “the condition in which a standard has been successfully established” according to one Free Online Dictionary at http://freedictionary.org/.

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