Finally the actual sample data points can be plotted to show their proximity to the line of perfect normally distribution and also to determine whether they fall within the upper and lower bands of the specified confidence interval.Ī histogram is probably a better tool to apply to make a quick determination of whether a data set might be normally distributed. Upper and lower confidence interval bands can also be added to that scatterplot. The line of perfect normal distribution for a given data set is fairly straightforward to construct on an Excel scatterplot. A Normal Probability Plot graphs the sample data values against the line of perfect normal distribution, i.e., the line that the data values would fall on if the data were perfectly normally distributed. data Z Score will be a perfectly straight line if the data are perfectly normally distributed. There is a linear relationship between data values and Z Scores of normally-distributed data. The Normal Probability Plot is based upon the following unique characteristic of the normal distribution: The wider that the confidence interval must be to contain the data points, the less likely it is that the data are normally distributed. Sample data points that do not fall on this line represent deviations from the normal distribution.Ĭonfidence interval bands can be added to a Normal Probability Plot to provide a visual representation of how wide a confidence interval would need to be to contain the majority of the sample data points if the data were normally distributed. Sample data are plotted against a line that the sample data would fall on if the sample was perfectly normally distributed. The Normal Probability Plot is a graphical tool used to determine whether sample data is approximately normally distributed.
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