The graphs also allow you to see the difference between one and two-sided (also called two-tailed) areas. Assuming the sample data was normally distributed, hereвЂ™s how the scores would look graphically:ДЄn interactive Graph of the Standard Normal Curve similar to Figures 1 & 2 is available for you to visualize how the z-scores and the area under the normal curve correspond. You can see where your score falls within the sample of other test takers and also see that the verbal score was better than the quantitative score. To convert these sigma values into a percentage you can look them up in a standard z-table, use the Excel formula =NORMSDIST(1.35) or use the Z-Score to Percentile Calculator (choose 1-sided) and get the percentages : 91% Verbal and 77% Quantitative. If you know the mean and standard deviations for a set of GRE test takers you can compare your scores.Д®TS publishes the means and standard deviations of a set of test takers on the GRE website.Д«y plugging in your scores you get the following: How good are these scores? Which is better, the Verbal or Quantitative score? Using a z-score can tell you how far you are from the mean and thus how well you performed. Both formulas essentially calculate the same thing:ДЇor example, lets say you took the GRE a few weeks ago and got scores of 630 Verbal and 700 Quantitative. Where x-bar and s are used as estimators for the populationвЂ™s true mean and standard deviation. Usability testing obviously samples a very small subset of the population and thus the following formula is used: The above formula is for obtaining a z-score for an entire population. If youвЂ™ve encountered the z-score in a statistics book you usually get some formula like: The short answer is: It depends on your data and what youвЂ™re looking for. The benefit of using a z-score in usability metrics was explained in вЂњ WhatвЂ™s a Z-Score and why use it in Usability Testing?вЂќ this article discusses different ways of calculating a z-score.

Xbar and s calculator