Symmetry plots can be used to assess whether sample data come from a symmetric distribution. Many statistical procedures assume that data come from a normal distribution. However, many procedures are robust to violations of normality, so having data from a symmetric distribution is often sufficient. Other procedures, such as nonparametric methods, assume symmetric distributions rather than normal distributions.
Therefore, a symmetry plot is a useful tool in many circumstances.
When use to Symmetry Plot
When the sample data follow a symmetric distribution, the X and Y coordinates will be approximately equal for all points and the data will fall in a straight line. Minitab draws a line on the plot to represent exact X-Y equality (a perfectly symmetric sample).
By comparing the data points to the line, you can assess the degree of symmetry present in the data. The more symmetric the data, the closer the points will be to the line. Even with normally distributed data, you can expect to see runs of points above or below the line. The important thing to look for is whether the points remain close to or parallel to the line, versus the points diverging from the line. You can detect the following asymmetric conditions.
Mark to Symmetry Plot
we will carify by sample of Minitab sample. Before doing further analyses, you would like to determine whether or not the sample data come from a symmetric
1.Open the worksheet EXH_QC.MTW.
3.Choose Stat > Quality Tools > Symmetry Plot.
4. In Variables, enter Faults. Click OK.
5. After that click OK that show Graph window output below.
Interpreting the results
Notice the few points above the line in the upper right corner. These points indicate skewness in the left tail of the
distribution. You can also see this skewness in the histogram.
Refer by www.minitab.com