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Summary
Description 
English: The redrawn chart comparing the various grading methods in a normal distribution. Includes: Standard deviations, cumulative percentages, percentile equivalents, Zscores and Tscores. Inspired by Figure 4.3 on Page 74 of Ward, A. W., MurrayWard, M. (1999). Assessment in the Classroom. Belmont, CA: Wadsworth. ISBN 0534527043 Note: The 95% range is labelled as 1.98 to +1.98 standard deviations. This is probably a typographic error, as the correct range is plus or minus 1.96 standard deviations.

Date 
20070712 (original upload date) 
Source 
Transferred from en.wikipedia to Commons by Abdull. 
Author 
Heds 1 at English Wikipedia 
Discussion
What is the zscore which has the steepest points of the curve? My guess it is z = 1, +1. The way to tell is to differentiate the pdf and find its maxima, but not sure if I'm up to that...
Is the 1.98 sigma/zscore for 95th percentile a typo? 1.96 is nearer 95% than 1.98 which corresponds to 95.2269%...
Y axis stands not for probability, as stated, but rather for probability density. Probability itself is zero for each given point. I think this is the important point. yes this plot is wrong !
Also, another comment: the Probability (Probability Density) cannot be negative (1.980 or 2.580) even though the X is negative.
If you want the probability within some interval, you would calculate the integral from one endpoint to the other. With the Normal Distribution, there is no elementary antiderivative, so the values are calculated using numerical methods. This is why you usually refer to a table that contains the calculated values. In order to use the tables you must first calculate the zscore.
Licensing
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