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1.Exploratory Data Analysis
1.3.EDA Techniques
1.3.5.Quantitative Techniques
 

1.3.5.14.

Anderson-Darling Test

Purpose:
Test for Distributional Adequacy 
The Anderson-Darling test (Stephens, 1974) is used to test if a sample of data comes from a specific distribution. It is a modification of the Kolmogorov-Smirnov (K-S) test and gives more weight to the tails than the K-S test. The K-S test is distribution free in the sense that the critical values do not depend on the specific distribution being tested. The Anderson-Darling test makes use of the specific distribution in calculating critical values. This has the advantage of allowing a more sensitive test and the disadvantage that critical values must be calculated for each distribution. Currently, tables of critical values are available for the normal, lognormal, exponential, Weibull, extreme value type I, and logistic distributions. We do not provide the tables of critical values in this handbook (see Stephens 1974, 1976, 1977, and 1979) since this test is usually applied with a statistical software program that will print the relevant critical values. 

The Anderson-Darling test is an alternative to the chi-square and Kolmogorov-Smirnov goodness of fit tests. 

Definition  The Anderson-Darling test is defined as: 
H0 The data follows a specified distribution. 
Ha The data do not follow the specified distribution 
Test Statistic:  The Anderson-Darling test statistic is defined as 

A**2 = -N - S

where 

S=SUM(i=1 to N)((2*i - 1)/N)*[LOG(F(Y(i)) + LOG(1 - F(Y(N+1-i))]

F is the cumulative distribution function of the specified distribution. 

Significance Level:  alpha
Critical Region:  The critical values for the Anderson-Darling test are dependent on the specific distribution that is being tested. Tabulated values and formulas have been published (Stephens, 1974, 1976, 1977, 1979) for a few specific distributions (normal, lognormal, exponential, Weibull, logistic, extreme value type 1). The test is a one-sided test and the hypothesis that the distribution is of a specific form is rejected if the test statistic, A, is greater than the critical value. 
Sample Output Dataplot generated the following output for the Anderson-Darling test. 1,000 random numbers were generated for a normal, double exponential, Cauchy, and lognormal distribution. In all four cases, the Anderson-Darling test was applied to test for a normal distribution. The test statistic show the characterstics of the test; where the data come from a normal distribution, the test statistic is small and the hypothesis accepted; where the data come from the double exponential, Cauchy, and lognormal distributions, the statistics are significant, and the hypothesis of an underlying normal distribution is rejected at significance levels of 0.10, 0.05, and 0.01. 

The normal random numbers were stored in the variable Y1, the double exponential random numbers were stored in the variable Y2, the Cauchy random numbers were stored in the variable Y3, and the lognormal random numbers were stored in the variable Y4. 

  
       ***************************************
       **  anderson darling normal test y1  **
       ***************************************
  
  
               ANDERSON DARLING 1-SAMPLE TEST
               THAT THE DATA COME FROM A NORMAL               DISTRIBUTION
  
 1. STATISTICS:
       NUMBER OF OBSERVATIONS                =     1000
       LOCATION PARAMETER                    =   0.4359940E-02
       SCALE PARAMETER                       =    1.001816
  
       ANDERSON DARLING TEST STATISTIC VALUE =   0.2566688
  
 2. CRITICAL VALUES:
       90         % POINT    =    1.062000
       95         % POINT    =    1.321000
       97.5       % POINT    =    1.591000
       99         % POINT    =    1.959000
  
 3. CONCLUSION (AT THE 5% LEVEL):
       THE DATA DO COME FROM A NORMAL              DISTRIBUTION.
  
  
       ***************************************
       **  anderson darling normal test y2  **
       ***************************************
  
  
               ANDERSON DARLING 1-SAMPLE TEST
               THAT THE DATA COME FROM A NORMAL               DISTRIBUTION
  
 1. STATISTICS:
       NUMBER OF OBSERVATIONS                =     1000
       LOCATION PARAMETER                    =   0.2034888E-01
       SCALE PARAMETER                       =    1.321627
  
       ANDERSON DARLING TEST STATISTIC VALUE =    5.827798
  
 2. CRITICAL VALUES:
       90         % POINT    =    1.062000
       95         % POINT    =    1.321000
       97.5       % POINT    =    1.591000
       99         % POINT    =    1.959000
  
 3. CONCLUSION (AT THE 5% LEVEL):
       THE DATA DO NOT COME FROM A NORMAL              DISTRIBUTION.
  
  
       ***************************************
       **  anderson darling normal test y3  **
       ***************************************
  
  
               ANDERSON DARLING 1-SAMPLE TEST
               THAT THE DATA COME FROM A NORMAL               DISTRIBUTION
  
 1. STATISTICS:
       NUMBER OF OBSERVATIONS                =     1000
       LOCATION PARAMETER                    =    1.503854
       SCALE PARAMETER                       =    35.13059
  
       ANDERSON DARLING TEST STATISTIC VALUE =    287.4414
  
 2. CRITICAL VALUES:
       90         % POINT    =    1.062000
       95         % POINT    =    1.321000
       97.5       % POINT    =    1.591000
       99         % POINT    =    1.959000
  
 3. CONCLUSION (AT THE 5% LEVEL):
       THE DATA DO NOT COME FROM A NORMAL              DISTRIBUTION.
  
  
       ***************************************
       **  anderson darling normal test y4  **
       ***************************************
  
  
               ANDERSON DARLING 1-SAMPLE TEST
               THAT THE DATA COME FROM A NORMAL               DISTRIBUTION
  
 1. STATISTICS:
       NUMBER OF OBSERVATIONS                =     1000
       LOCATION PARAMETER                    =    1.518372
       SCALE PARAMETER                       =    1.719969
  
       ANDERSON DARLING TEST STATISTIC VALUE =    81.64876
  
 2. CRITICAL VALUES:
       90         % POINT    =    1.062000
       95         % POINT    =    1.321000
       97.5       % POINT    =    1.591000
       99         % POINT    =    1.959000
  
 3. CONCLUSION (AT THE 5% LEVEL):
       THE DATA DO NOT COME FROM A NORMAL              DISTRIBUTION.
  
      
Interpretation of the Sample Output The output is divided into three sections.
  1. The first section prints the number of observations and estimates for the location and scale parameters
  2. The second section prints the upper critical value for the Anderson-Darling test statistic distribution corresponding to various significance levels. The value in the first column, the confidence level of the test, is equivalent to 100(1-alpha). We reject the null hypothesis at that significance level if the value of the Anderson-Darling test statistic printed in section one is greater than the critical value printed in the last column. 
  3. The third section prints the conclusion for a 95% test. For a different significance level, the appropriate conclusion can be drawn from the table printed in section two. For example, for  = 0.10, we look at the row for the 90% confidence and compare the critical value 1.062 to the Anderson-Darling test statistic (for the normal data) 0.256. Since the test statistic is less than the critical value, we do not reject the null hypothesis at the alpha = 0.10 level.
As we would hope, the Anderson-Darling accepts the hypothesis of normality for the normal random numbers and rejects it for the 3 non-normal cases.

The output from other statistical software programs may look somewhat different than the output above. 

Questions  The Anderson-Darling test can be used to answer the following questions: 
  • Is the data from a normal distribution? 
  • Is the data from a log-normal distribution? 
  • Is the data from a Weibull distribution? 
  • Is the data from a exponential distribution? 
  • Is the data from a logistic distribution? 
Importance  Many statistical tests and procedures are based on specific distributional assumptions. The assumption of normality is particularly common in classical statistical tests. Much reliability modeling is based on the assumption that the data follow a Weibull distribution. 

There are many non-parametric and robust techniques that do not make strong distributional assumptions. However, techniques based on specific distributional assumptions are in general more powerful than non-parametric and robust techniques. Therefore, if the distributional assumptions can be validated, they are generally preferred. 

Related Techniques Chi-Square Goodness of Fit Test
Kolmogorov-Smirnov Test
Shapiro-Wilk Normality Test
Probability Plot
Probability Plot Correlation Coefficient Plot
Case Study Airplane glass failure time data. 
Software The Anderson-Darling goodness of fit test is available in some general purpose statistical software programs, including Dataplot
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