Checking if data follows a normal distribution
The normality test in basic statistics is a tool that is similar to the probability plot used in Chapter 2, Tables and Graphs. It is limited to testing normality but it does offer us the choice between Anderson-Darling statistics, Kolmogorov-Smirnov, and Ryan-Joiner tests.
Here, we will use the data from the Oxford weather station and check the normality of the mean maximum temperature for July.
Getting ready
The data for this example can be found on the Met Office website at http://www.metoffice.gov.uk/climate/uk/stationdata/oxforddata.txt.
The data is made available in the Oxford Weather.txtfile; this preserves the format from the website and also the Minitab Oxford weather (cleaned).mtwfile. This worksheet is correctly imported into Minitab for us. Follow steps 1 to 12 in the Producing a graphical summary of data recipe to import the results.
How to do it…
- Go to the Data menu and select Split Worksheet….
- In the By variables section, enter the column for month (C2 mm).
- Click on OK.
- From the new worksheets, select the worksheet for July.
- Go to the Stat menu and then Basic Statistics and click on Normality Test….
- In the dialog box, enter the column for mean maximum temperature in the Variable field.
- Click on OK.
How it works…
The output will give us means, standard deviation, the test statistic, and the P-value. The Anderson-Darling test is more sensitive to deviations from normality in the tails of the distribution.