A Normal Probability Plot (NPP) is a graphical technique for assessing whether a dataset is approximately normally distributed. The plot is constructed by plotting the quantiles of the dataset against the quantiles of a standard normal distribution. If the points on the plot form a roughly straight line, it indicates that the data follows a normal distribution.
To create an NPP, the data is first sorted in ascending order. The expected normal quantiles are calculated based on the mean and standard deviation of the dataset. These expected values are then plotted against the actual data values. Points that deviate significantly from the line may indicate outliers or that the data does not follow a normal distribution.
NPPs are particularly useful in statistics and data analysis, as many statistical tests and methods assume normality of the data. By visually inspecting the NPP, analysts can make informed decisions about the appropriate statistical techniques to apply. In cases where the data does not follow a normal distribution, alternative methods or transformations may be necessary to meet the assumptions of the analysis.