Interpret the key results for Outlier Test

Complete the following steps to interpret an outlier test. Key output includes the p-value, the outlier, and the outlier plot.

In This Topic

Step 1: Determine whether an outlier exists
Step 2: Determine the value of the outlier
Step 3: Visually identify the outlier

Step 1: Determine whether an outlier exists

To determine whether an outlier exists, compare the p-value to the significance level. Usually, a significance level (denoted as α or alpha) of 0.05 works well. A significance level of 0.05 indicates a 5% risk of concluding that an outlier exists when no actual outlier exists.

P-value ≤ α: An outlier exists (Reject H₀): If the p-value is less than or equal to the significance level, the decision is to reject the null hypothesis and conclude that an outlier exists. Try to identify the cause of any outliers. Correct any data–entry errors or measurement errors. Consider removing data values that are associated with abnormal, one-time events (special causes).
P-value > α: You cannot conclude an outlier exists (Fail to reject H₀): If the p-value is greater than the significance level, the decision is to fail to reject the null hypothesis because you do not have enough evidence to conclude that an outlier exists. You should make sure that your test has enough power to detect an outlier. For more information, go to Increase power.

Method

Null hypothesis	All data values come from the same normal population
Alternative hypothesis	Smallest data value is an outlier
Significance level	α = 0.05

Grubbs' Test

Variable	N	Mean	StDev	Min	Max	G	P
BreakStrength	14	123.4	46.3	12.4	193.1	2.40	0.044

Outlier

Variable	Row	Outlier
BreakStrength	10	12.38

Key Result: P

In these results, the null hypothesis states that all data values come from the same normal population. Because the p-value is 0.044, which is less than the significance level of 0.05, the decision is to reject the null hypothesis and conclude that an outlier exists.

Step 2: Determine the value of the outlier

If the test identifies an outlier in the data, then Minitab displays an outlier table. Use the outlier table to determine the value of the outlier, and the row in the worksheet that contains the outlier.

Method

Null hypothesis	All data values come from the same normal population
Alternative hypothesis	Smallest data value is an outlier
Significance level	α = 0.05

Grubbs' Test

Variable	N	Mean	StDev	Min	Max	G	P
BreakStrength	14	123.4	46.3	12.4	193.1	2.40	0.044

Outlier

Variable	Row	Outlier
BreakStrength	10	12.38

Key Results: Row, Outlier

In these results, the value of the outlier is 12.38, and it is in row 10.

Step 3: Visually identify the outlier

Use the outlier plot to visually identify an outlier in the data. If an outlier exists, Minitab represents it on the plot as a red square. Try to identify the cause of any outliers. Correct any data–entry errors or measurement errors. Consider removing data values for abnormal, one-time events (also called special causes).

In this outlier plot, the smallest value, 12.38, is an outlier.