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To add a reference line use the qqline function directly after the qqnorm function as shown. Order your n number of points of raw data from the minimum value to the maximum observed values.

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With a set of data from a process or product characteristic you re ready to begin the steps to creating a normal probability plot.

How to plot normal probability plot. You can create a normal probability plot using the qqnorm function. You can also add a reference line to the plot which makes it easier to determine whether the data points are falling into a straight line. With this type of graph z scores are plotted against your data set.

First the x axis is transformed so that a cumulative normal density function will plot in a straight line. We will demonstrate the procedure using the data below. A normal probability plot can be used to determine if small sets of data come from a normal distribution.

Assign a rank order number i to each of the n points of data. Assuming we have two distributions f and g and a point of evaluation z any value the point on the plot indicates what percentage of data lies at or below z in both f and g as per definition of the cdf. We will eventually make a plot that we hope is linear.

The normal percent point function the g is simply replaced by the percent point function of the desired distribution. Where the mean is zero and the standard deviation is one. Normal probability plot in minitab.

Probability plots for distributions other than the normal are computed in exactly the same way. Interpretation of the points on the plot. A straight line in a normal probability plot indicates your data does fit a normal probability distribution.

That is a probability plot can easily be generated for any distribution for which you have the percent point function. A normal probability plot is one way you can tell if data fits a normal distribution a bell curve. This involves using the probability properties of the normal distribution.

Then using the mean and standard deviation sigma which are calculated from the data the data is transformed to the standard normal values i e. To compare the distributions we check if the points lie on a 45 degree line x y.