scipy.stats.trapezoid

scipy.stats.trapezoid = <scipy.stats._continuous_distns.trapezoid_gen object>

A trapezoidal continuous random variable.

As an instance of the rv_continuous class, trapezoid object inherits from it a collection of generic methods (see below for the full list), and completes them with details specific for this particular distribution.

Notes

The trapezoidal distribution can be represented with an up-sloping line from loc to (loc + c*scale), then constant to (loc + d*scale) and then downsloping from (loc + d*scale) to (loc+scale). This defines the trapezoid base from loc to (loc+scale) and the flat top from c to d proportional to the position along the base with 0 <= c <= d <= 1. When c=d, this is equivalent to triang with the same values for loc, scale and c. The method of [1] is used for computing moments.

trapezoid takes \(c\) and \(d\) as shape parameters.

The probability density above is defined in the “standardized” form. To shift and/or scale the distribution use the loc and scale parameters. Specifically, trapezoid.pdf(x, c, d, loc, scale) is identically equivalent to trapezoid.pdf(y, c, d) / scale with y = (x - loc) / scale. Note that shifting the location of a distribution does not make it a “noncentral” distribution; noncentral generalizations of some distributions are available in separate classes.

The standard form is in the range [0, 1] with c the mode. The location parameter shifts the start to loc. The scale parameter changes the width from 1 to scale.

References

1

Kacker, R.N. and Lawrence, J.F. (2007). Trapezoidal and triangular distributions for Type B evaluation of standard uncertainty. Metrologia 44, 117-127. DOI:10.1088/0026-1394/44/2/003

Examples

>>> from scipy.stats import trapezoid
>>> import matplotlib.pyplot as plt
>>> fig, ax = plt.subplots(1, 1)

Calculate the first four moments:

>>> c, d = 0.2, 0.8
>>> mean, var, skew, kurt = trapezoid.stats(c, d, moments='mvsk')

Display the probability density function (pdf):

>>> x = np.linspace(trapezoid.ppf(0.01, c, d),
...                 trapezoid.ppf(0.99, c, d), 100)
>>> ax.plot(x, trapezoid.pdf(x, c, d),
...        'r-', lw=5, alpha=0.6, label='trapezoid pdf')

Alternatively, the distribution object can be called (as a function) to fix the shape, location and scale parameters. This returns a “frozen” RV object holding the given parameters fixed.

Freeze the distribution and display the frozen pdf:

>>> rv = trapezoid(c, d)
>>> ax.plot(x, rv.pdf(x), 'k-', lw=2, label='frozen pdf')

Check accuracy of cdf and ppf:

>>> vals = trapezoid.ppf([0.001, 0.5, 0.999], c, d)
>>> np.allclose([0.001, 0.5, 0.999], trapezoid.cdf(vals, c, d))
True

Generate random numbers:

>>> r = trapezoid.rvs(c, d, size=1000)

And compare the histogram:

>>> ax.hist(r, density=True, histtype='stepfilled', alpha=0.2)
>>> ax.legend(loc='best', frameon=False)
>>> plt.show()

Methods

rvs(c, d, loc=0, scale=1, size=1, random_state=None)	Random variates.
pdf(x, c, d, loc=0, scale=1)	Probability density function.
logpdf(x, c, d, loc=0, scale=1)	Log of the probability density function.
cdf(x, c, d, loc=0, scale=1)	Cumulative distribution function.
logcdf(x, c, d, loc=0, scale=1)	Log of the cumulative distribution function.
sf(x, c, d, loc=0, scale=1)	Survival function (also defined as 1 - cdf, but sf is sometimes more accurate).
logsf(x, c, d, loc=0, scale=1)	Log of the survival function.
ppf(q, c, d, loc=0, scale=1)	Percent point function (inverse of cdf — percentiles).
isf(q, c, d, loc=0, scale=1)	Inverse survival function (inverse of sf).
moment(n, c, d, loc=0, scale=1)	Non-central moment of order n
stats(c, d, loc=0, scale=1, moments=’mv’)	Mean(‘m’), variance(‘v’), skew(‘s’), and/or kurtosis(‘k’).
entropy(c, d, loc=0, scale=1)	(Differential) entropy of the RV.
fit(data)	Parameter estimates for generic data. See scipy.stats.rv_continuous.fit for detailed documentation of the keyword arguments.
expect(func, args=(c, d), loc=0, scale=1, lb=None, ub=None, conditional=False, **kwds)	Expected value of a function (of one argument) with respect to the distribution.
median(c, d, loc=0, scale=1)	Median of the distribution.
mean(c, d, loc=0, scale=1)	Mean of the distribution.
var(c, d, loc=0, scale=1)	Variance of the distribution.
std(c, d, loc=0, scale=1)	Standard deviation of the distribution.
interval(alpha, c, d, loc=0, scale=1)	Endpoints of the range that contains fraction alpha [0, 1] of the distribution

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