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Legacy Tools › ArcView 3.x Extensions › Quantile Functions

Quantile Functions

In general, the Quantile Functions (sometimes referred to as Inverse Density Functions or Percent Point Functions) return the Value X at which P(X) = [specified cumulative probability], given that particular distribution.

Distributions
Beta Chi Square Logistic Poisson
Binomial Exponential LogNormal Student's T
Cauchy F Normal Weibull
  1. IDF_Beta:  This function takes the specified probability and returns the value X, such that P(X) = P-value, given the Beta distribution with the two specified Shape parameters. Because the formula for this function does not exist in a closed form, it must be computed numerically. This script arrives at the X-value through an iterative process, repeatedly testing X-values with the CDF_Beta function until it arrives at P value that is within 1x10-12 units from the specified P-value (this usually takes between 30-60 iterations).

    a) Parameters:

    1. P-value: Number (0 >= p >= 1) 
    2. Shape1: Number > 0
    3. Shape2: Number > 0

    b) Usages:

    1. From "Probability Distribution Calculator", select "Quantile (IDF)" and Beta distribution.
    2. (Avenue): theProb = av.Run("Jennessent.DistFunc”, {“IDF_Beta”, {P-value, Shape1, Shape2}})
    3. (Avenue): theProb = av.Run("Jennessent.TableDistFunc”, {“IDF_Beta”, {P-value, Shape1, Shape2}})
    4. (Avenue): theProb = av.Run("Jennessent.IDF_Beta", {P-value, Shape1, Shape2})
       

c) Function:
  1. IDF_Binomial:  This function takes the specified probability and returns the value X such that the Probability of getting (X – 1) successes <= the Specified Probability. This function takes an iterative approach to finding the correct X value, repeatedly trying different values of X until it reaches the correct one. This iterative process rarely takes more than 25 repetitions.

    a) Parameters:

    1. P-value: Number (0 >= p >= 1) 
    2. # Trials: Integer >= 2, # Successes
    3. Probability of Success: Number (0 >= p >= 1)

    b) Usages:

    1. From "Probability Distribution Calculator", select "Quantile (IDF)" and Binomial distribution.
    2. (Avenue): theProb = av.Run("Jennessent.DistFunc”, {“IDF_Binomial”, {P-value, #Trials, Probability of Success}})
    3. (Avenue): theProb = av.Run("Jennessent.TableDistFunc”, {“IDF_Binomial”, {P-value, #Trials, Probability of Success}})
    4. (Avenue): theProb = av.Run("Jennessent.IDF_Binomial", {P-value, #Trials, Probability of Success})
       

c) Function:

 

  • IDF_Cauchy:  This function takes the specified probability and returns the value X, such that P(X) = P-value, given the Cauchy distribution with the specified location and scale parameters. The Standardized Cauchy distribution has Location = 0 and Scale = 1.

    a) Parameters:

    1. P-value: Number (0 >= p >= 1) 
    2. Location: Number
    3. Scale: Number > 0

    b) Usages:

    1. From "Probability Distribution Calculator", select "Quantile (IDF)" and Cauchy distribution.
    2. (Avenue): theProb = av.Run("Jennessent.DistFunc”, {“IDF_Cauchy”, {P-value, Location, Scale}})
    3. (Avenue): theProb = av.Run("Jennessent.TableDistFunc”, {“IDF_Cauchy”, {P-value, Location, Scale}})
    4. (Avenue): theProb = av.Run("Jennessent.IDF_Cauchy", {P-value, Location, Scale})
       

    • c) Function:

     

  • IDF_ChiSquare:  This function takes the specified probability and returns the value X, such that P(X) = P-value, given the Chi-Square distribution with the specified Degrees of Freedom. Because the formula for this function does not exist in a closed form, it must be computed numerically. This script arrives at the X-value through an iterative process, repeatedly testing X-values with the CDF_ChiSquare function until it arrives at P value that is within 1x10 -12 units from the specified P-value (this usually takes between 30-60 iterations). The Chi-Square distribution results when v (where v = Degrees of Freedom) independent variables with standard normal distributions are squared and summed (Croarkin & Tobias, Date unknown).

    a) Parameters:

    1. P-value: Number (0 >= p >= 1)
    2. Degrees of Freedom: Number > 0

    b) Usages:

    1. From "Probability Distribution Calculator", select "Quantile (IDF)" and Chi Square distribution.
    2. (Avenue): theProb = av.Run("Jennessent.DistFunc”, {“IDF_ChiSquare”, {P-value, DF}})
    3. (Avenue): theProb = av.Run("Jennessent.TableDistFunc”, {“IDF_ChiSquare”, {P-value, DF}})
    4. (Avenue): theProb = av.Run("Jennessent.IDF_ChiSquare", {P-value, DF})
       

    • c) Function:

     

  • IDF_Exp:  This function takes the specified probability and returns the value X, such that P(X) = P-value, given the Exponential distribution with the specified mean. This script uses the 1-parameter version of the equation (i.e. assuming Location = 0). The Standard Exponential Distribution is that which has Mean = 1.

    a) Parameters:

    1. P-value: Number (0 >= p >= 1)
    2. Mean: Number > 0

    b) Usages:

    1. From "Probability Distribution Calculator", select "Quantile (IDF)" and Exponential distribution.
    2. (Avenue): theProb = av.Run("Jennessent.DistFunc”, {“IDF_Exp”, {P-value, Mean}})
    3. (Avenue): theProb = av.Run("Jennessent.TableDistFunc”, {“IDF_Exp”, {P-value, Mean}})
    4. (Avenue): theProb = av.Run("Jennessent.IDF_Exp", {P-value, Mean})
       

    • c) Function:

     

  • IDF_F:  This function takes the specified probability and returns the value X, such that P(X) = P-value, given the F distribution with the specified Degrees of Freedom. Because the formula for this function does not exist in a closed form, it must be computed numerically. This script arrives at the X-value through an iterative process, repeatedly testing X-values with the CDF_F function until it arrives at P value that is within 1x10-12 units from the specified P-value (this usually takes between 30-60 iterations). The F distribution is the ratio of two Chi Square distributions with ratios v1 and v2 respectively.

    a) Parameters:

    1. P-value: Number (0 >= p >= 1) 
    2. 1st Degrees of Freedom: Number > 1
    3. 2nd Degrees of Freedom: Number > 1

    b) Usages:

    1. From "Probability Distribution Calculator", select "Quantile (IDF)" and F distribution.
    2. (Avenue): theProb = av.Run("Jennessent.DistFunc”, {“IDF_F”, {P-value, DF1, DF2}})
    3. (Avenue): theProb = av.Run("Jennessent.TableDistFunc”, {“IDF_F”, {P-value, DF1, DF2}})
    4. (Avenue): theProb = av.Run("Jennessent.IDF_F", {P-value, DF1, DF2})
       

    • c) Function:

     

  • IDF_Logistic:  This function takes the specified probability and returns the value X, such that P(X) = P-value, given the Logistic distribution with the specified mean and scale parameters.

    a) Parameters:

    1. P-value: Number (0 >= p >= 1)
    2. Mean: Number
    3. Scale: Number > 0

    b) Usages:

    1. From "Probability Distribution Calculator", select "Quantile (IDF)" and Logistic distribution.
    2. (Avenue): theProb = av.Run("Jennessent.DistFunc”, {“IDF_Logistic”, {P-value, Mean, Scale}})
    3. (Avenue): theProb = av.Run("Jennessent.TableDistFunc”, {“IDF_Logistic”, {P-value, Mean, Scale}})
    4. (Avenue): theProb = av.Run("Jennessent.IDF_Logistic", {P-value, Mean, Scale})
       

    • c) Function:

     

  • IDF_LogNormal:  This function takes the specified probability and returns the value X, such that P(X) = P-value, given the LogNormal distribution with the specified mean and scale parameters. Because the formula for this function does not exist in a closed form, it must be computed numerically. This script arrives at the X-value through an iterative process, repeatedly testing X-values with the CDF_LogNormal function until it arrives at P value that is within 1x10-12 units from the specified P-value (this usually takes between 30-60 iterations). A LogNormal distribution occurs when natural logarithms of variable X are normally distributed. The Standard LogNormal Distribution is that with Mean = 0 and Scale = 1. The 2-Parameter LogNormal Distribution is that with Mean = 0.

    a) Parameters:

    1. P-value: Number (0 >= p >= 1)
    2. Mean: Number > 0
    3. Scale: Number > 0

    b) Usages:

    1. From "Probability Distribution Calculator", select "Quantile (IDF)" and LogNormal distribution.
    2. (Avenue): theProb = av.Run("Jennessent.DistFunc”, {“IDF_LogNormal, {P-value, Mean, Scale}})
    3. (Avenue): theProb = av.Run("Jennessent.TableDistFunc”, {“IDF_LogNormal, {P-value, Mean, Scale}})
    4. (Avenue): theProb = av.Run("Jennessent.IDF_LogNormal", {P-value, Mean, Scale})
       

    • c) Function:

     

  • IDF_Normal:  This function takes the specified probability and returns the value X, such that P(X) = P-value, given the Normal distribution with the specified mean and standard deviation. Because the formula for this function does not exist in a closed form, it must be computed numerically. This script arrives at the X-value through an iterative process, repeatedly testing X-values with the CDF_Normal_Simpsons function until it arrives at P value that is within 1x10-12 units from the specified P-value (this usually takes between 30-60 iterations). Furthermore, there is no closed formula for calculating the Normal cumulative distribution function, so this script uses the Simpson’s approximation method (Stewart 1998, p. 421-424) to calculate a highly accurate estimate (accuracy to > 12 decimal places). The Standard Normal Distribution is that with Mean = 0 and Standard Deviation = 1.

    a) Parameters:

    1. P-value: Number (0 >= p >= 1)
    2. Mean: Number
    3. Standard Deviation: Number > 0

    b) Usages:

    1. From "Probability Distribution Calculator", select "Quantile (IDF)" and Normal distribution.
    2. (Avenue): theProb = av.Run("Jennessent.DistFunc”, {“IDF_Normal_Simpsons, {P-value, Mean, St. Dev.}})
    3. (Avenue): theProb = av.Run("Jennessent.TableDistFunc”, {“IDF_Normal_Simpsons, {P-value, Mean, St. Dev.}})
    4. (Avenue): theProb = av.Run("Jennessent.IDF_Normal_Simpsons", {P-value, Mean, St. Dev.})
       

    • c) Function:

     

  • IDF_Poisson: This function takes the specified probability and returns the value X such that the Probability of getting (X – 1) events <= the Specified Probability. This function takes an iterative approach to finding the correct X value, repeatedly trying different values of X until it reaches the correct one.

    a) Parameters:

    1. P-value: Number (0 >= p >= 1)
    2. Mean: Number > 0

    b) Usages:

    1. From "Probability Distribution Calculator", select "Quantile (IDF)" and Poisson distribution.
    2. (Avenue): theProb = av.Run("Jennessent.DistFunc”, {“IDF_Poisson, {#P-value, Mean}})
    3. (Avenue): theProb = av.Run("Jennessent.TableDistFunc”, {“IDF_Poisson, {P-value, Mean}})
    4. (Avenue): theProb = av.Run("Jennessent.IDF_Poisson", {P-value, Mean})
       

    • c) Function:

     

  • IDF_StudentsT:  This function takes the specified probability and returns the value X, such that P(X) = P-value, given the Student’s T distribution with the specified Degrees of Freedom. Because the formula for this function does not exist in a closed form, it must be computed numerically. This script arrives at the X-value through an iterative process, repeatedly testing X-values with the CDF_StudentsT function until it arrives at P value that is within 1x10-12 units from the specified P-value (this usually takes between 30-60 iterations). A Student’s T distribution with 1df is a Cauchy Distribution, and it approaches a Normal distribution when DF>30. Various sources recommend using the Normal distribution if DF>100.

    a) Parameters:

    1. P-value: Number (0 >= p >= 1)
    2. Degrees of Freedom: Number > 0

    b) Usages:

    1. From "Probability Distribution Calculator", select "Quantile (IDF)" and Student’s T distribution.
    2. (Avenue): theProb = av.Run("Jennessent.DistFunc”, {“IDF_StudentsT, {P-value, DF}})
    3. (Avenue): theProb = av.Run("Jennessent.TableDistFunc”, {“IDF_StudentsT, {P-value, DF}})
    4. (Avenue): theProb = av.Run("Jennessent.IDF_StudentsT", {P-value, DF})

    c) Function: The IDF_StudentsT T Function is dependent on whether the test value is positive or negative:

    •  

  • IDF_Weibull:  This function takes the specified probability and returns the value X, such that P(X) = P-value, given the Weibull distribution with the specified Location, Scale and Shape parameters. The Standardized Weibull Distribution is that with Location = 0 and Scale = 1. The 2-Parameter Weibull Distribution is that with Location = 0.

    a) Parameters:

    1. P-value: Number (0 >= p >= 1)
    2. Location: Number
    3. Scale: Number > 0
    4. Shape: Number > 0

    b) Usages:

    1. From "Probability Distribution Calculator", select "Quantile (IDF)" and Weibull distribution.
    2. (Avenue): theProb = av.Run("Jennessent.DistFunc”, {“IDF_Weibull, {P-value, Location, Scale, Number}})
    3. (Avenue): theProb = av.Run("Jennessent.TableDistFunc”, {“IDF_Weibull, {P-value, Location, Scale, Number}})
    4. (Avenue): theProb = av.Run("Jennessent.IDF_Weibull", {P-value, Location, Scale, Number})
       

    • c) Function:

    Back to Statistics/Distributions | Summary Statistics | Probability Calculators | References

    Calculating Summary Statistics with Avenue

    Discussion of Distribution Functions:
    Probability Density Functions | Cumulative Distribution Functions | Quantile Functions

    Download Statistics Extension Manual (Adobe PDF, version 5)

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