skewness.js (1923B)
1 /** 2 * @license Apache-2.0 3 * 4 * Copyright (c) 2018 The Stdlib Authors. 5 * 6 * Licensed under the Apache License, Version 2.0 (the "License"); 7 * you may not use this file except in compliance with the License. 8 * You may obtain a copy of the License at 9 * 10 * http://www.apache.org/licenses/LICENSE-2.0 11 * 12 * Unless required by applicable law or agreed to in writing, software 13 * distributed under the License is distributed on an "AS IS" BASIS, 14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 15 * See the License for the specific language governing permissions and 16 * limitations under the License. 17 */ 18 19 'use strict'; 20 21 // MODULES // 22 23 var isnan = require( '@stdlib/math/base/assert/is-nan' ); 24 var pow = require( '@stdlib/math/base/special/pow' ); 25 var SQRT2 = require( '@stdlib/constants/float64/sqrt-two' ); 26 27 28 // MAIN // 29 30 /** 31 * Returns the skewness of a triangular distribution. 32 * 33 * @param {number} a - minimum support 34 * @param {number} b - maximum support 35 * @param {number} c - mode 36 * @returns {number} skewness 37 * 38 * @example 39 * var v = skewness( 0.0, 1.0, 0.5 ); 40 * // returns 0.0 41 * 42 * @example 43 * var v = skewness( 4.0, 12.0, 9.0 ); 44 * // returns ~-0.236 45 * 46 * @example 47 * var v = skewness( -4.0, 4.0, -1.0 ); 48 * // returns ~0.236 49 * 50 * @example 51 * var v = skewness( 1.0, -0.1, 0.5 ); 52 * // returns NaN 53 * 54 * @example 55 * var v = skewness( 0.0, 1.0, 2.0 ); 56 * // returns NaN 57 * 58 * @example 59 * var v = skewness( NaN, 4.0, 2.0 ); 60 * // returns NaN 61 * 62 * @example 63 * var v = skewness( 0.0, NaN, 2.0 ); 64 * // returns NaN 65 * 66 * @example 67 * var v = skewness( 0.0, 4.0, NaN ); 68 * // returns NaN 69 */ 70 function skewness( a, b, c ) { 71 var out; 72 if ( 73 isnan( a ) || 74 isnan( b ) || 75 isnan( c ) || 76 !( a <= c && c <= b ) 77 ) { 78 return NaN; 79 } 80 out = SQRT2 * ( a + b - (2*c) ) * ( (2*a) - b - c ) * ( a - (2*b) + c ); 81 out /= 5.0 * pow( (a*a) + (b*b) + (c*c) - (a*b) - (a*c) - (b*c), 1.5 ); 82 return out; 83 } 84 85 86 // EXPORTS // 87 88 module.exports = skewness;