function Beta() {
	var alpha;
	var beta;
	if ( !(this instanceof Beta) ) {
		if ( arguments.length === 0 ) {
			return new Beta();
		}
		return new Beta( arguments[ 0 ], arguments[ 1 ] );
	}
	if ( arguments.length ) {
		alpha = arguments[ 0 ];
		beta = arguments[ 1 ];
		if ( !isPositive( alpha ) ) {
			throw new TypeError( format( 'invalid argument. First shape parameter must be a positive number. Value: `%s`.', alpha ) );
		}
		if ( !isPositive( beta ) ) {
			throw new TypeError( format( 'invalid argument. Second shape parameter must be a positive number. Value: `%s`.', beta ) );
		}
	} else {
		alpha = 1.0;
		beta = 1.0;
	}
	defineProperty( this, 'alpha', {
		'configurable': false,
		'enumerable': true,
		'get': function get() {
			return alpha;
		},
		'set': function set( value ) {
			if ( !isPositive( value ) ) {
				throw new TypeError( format( 'invalid assignment. Must be a positive number. Value: `%s`.', value ) );
			}
			alpha = value;
		}
	});
	defineProperty( this, 'beta', {
		'configurable': false,
		'enumerable': true,
		'get': function get() {
			return beta;
		},
		'set': function set( value ) {
			if ( !isPositive( value ) ) {
				throw new TypeError( format( 'invalid assignment. Must be a positive number. Value: `%s`.', value ) );
			}
			beta = value;
		}
	});
	return this;
}
Beta distribution constructor.

@constructor
@param {PositiveNumber} [alpha=1.0] - first shape parameter
@param {PositiveNumber} [beta=1.0] - second shape parameter
@throws {TypeError} `alpha` must be a positive number
@throws {TypeError} `beta` must be a positive number
@returns {Beta} distribution instance

@example
var beta = new Beta( 1.0, 1.0 );

var y = beta.cdf( 0.8 );
// returns 0.8

var mu = beta.mean;
// returns 0.5