wblplot

Weibull probability plot

Syntax

wblplot(x)

wblplot(ax,x)

h = wblplot(___)

Description

wblplot(x) creates a Weibull probability plot comparing the distribution of the data in x to the Weibull distribution.

wblplot plots each data point in x using plus sign ('+') markers and draws two reference lines that represent the theoretical distribution. A solid reference line connects the first and third quartiles of the data, and a dashed reference line extends the solid line to the ends of the data. If the sample data has a Weibull distribution, then the data points appear along the reference line. A distribution other than Weibull introduces curvature in the data plot.

wblplot(ax,x) adds a Weibull probability plot into the axes specified by ax.

h = wblplot(___) returns graphics handles corresponding to the plotted lines, using any of the input arguments in the previous syntaxes.

Examples

Create Weibull Probability Plot

Generate a vector r containing 50 random numbers from the Weibull distribution with the scale parameter 1.2 and the shape parameter 1.5.

rng('default')  % For reproducibility
r = wblrnd(1.2,1.5,50,1);

Create a Weibull probability plot to visually determine if the data comes from a Weibull distribution.

wblplot(r)

The plot indicates that the data likely comes from a Weibull distribution.

Test for Weibull Distribution

Generate two sample data sets, one from a Weibull distribution and another from a lognormal distribution. Perform the Lilliefors test to assess whether each data set is from a Weibull distribution. Confirm the test decision by performing a visual comparison using a Weibull probability plot (wblplot).

Generate samples from a Weibull distribution.

rng('default')
data1 = wblrnd(0.5,2,[500,1]);

Perform the Lilliefors test by using the lillietest. To test data for a Weibull distribution, test if the logarithm of the data has an extreme value distribution.

h1 = lillietest(log(data1),'Distribution','extreme value')

h1 = 
0

The returned value of h1 = 0 indicates that lillietest fails to reject the null hypothesis at the default 5% significance level. Confirm the test decision using a Weibull probability plot.

wblplot(data1)

The plot indicates that the data follows a Weibull distribution.

Generate samples from a lognormal distribution.

data2 =lognrnd(5,2,[500,1]);

Perform the Lilliefors test.

h2 = lillietest(log(data2),'Distribution','extreme value')

h2 = 
1

The returned value of h2 = 1 indicates that lillietest rejects the null hypothesis at the default 5% significance level. Confirm the test decision using a Weibull probability plot.

wblplot(data2)

The plot indicates that the data does not follow a Weibull distribution.

Input Arguments

x — Sample data

numeric vector | numeric matrix

Sample data, specified as a numeric vector or numeric matrix. wblplot displays each value in x using the symbol '+'. If x is a matrix, then wblplot displays a separate line for each column of x.

Data Types: single | double

ax — Target axes

Axes object | UIAxes object

Target axes, specified as an Axes object or a UIAxes object. wblplot adds an additional plot into the axes specified by ax. For details, see Axes Properties and UIAxes Properties.

Use gca to return the current axes for the current figure.

Output Arguments

h — Graphics handles for line objects

vector of Line graphics handles

Graphics handles for line objects, returned as a vector of Line graphics handles. Graphics handles are unique identifiers that you can use to query and modify the properties of a specific line on the plot. For each column of x, wblplot returns three handles:

• The line representing the data points. wblplot represents each data point in x using plus sign ('+') markers.

• The line joining the first and third quartiles of each column of x, represented as a solid line.

• The extrapolation of the quartile line, extended to the minimum and maximum values of x, represented as a dashed line.

To view and set properties of line objects, use dot notation. For information on using dot notation, see Access Property Values. For information on the Line properties that you can set, see Line Properties.

Algorithms

wblplot matches the quantiles of sample data to the quantiles of a Weibull distribution. The sample data is sorted, scaled logarithmically, and plotted on the x-axis. The y-axis represents the quantiles of the Weibull distribution, converted into probability values. Therefore, the y-axis scaling is not linear.

Where the x-axis value is the ith sorted value from a sample of size N, the y-axis value is the midpoint between evaluation points of the empirical cumulative distribution function of the data. The midpoint is equal to $$\frac{\left(i-0.5\right)}{N}$$.

wblplot superimposes a reference line to assess the linearity of the plot. The line goes through the first and third quartiles of the data.

Alternative Functionality

You can use the probplot function to create a probability plot. The probplot function enables you to indicate censored data and specify the distribution for a probability plot.

Version History

Introduced before R2006a