code: function iqr( arr, opts ) {
  	if ( !Array.isArray( arr ) ) {
  		throw new TypeError( 'iqr()::invalid input argument. Must provide an array.' );
  	}
  	if ( arguments.length > 1 ) {
  		if ( !isObject( opts ) ) {
  			throw new TypeError( 'iqr()::invalid input argument. Options should be an object.' );
  		}
  	} else {
  		opts = {
  			'sorted': false
  		};
  	}
  	if ( !opts.sorted ) {
  		arr = arr.slice();
  		arr.sort( ascending );
  		opts.sorted = true;
  	}
  	return quantile( arr, 0.75, opts ) - quantile( arr, 0.25, opts );
  }
docs: INTERQUARTILE RANGE // 
  FUNCTION: iqr( arr )
  Computes the interquartile range for an array.
  
  @param {Array} arr - array of values
  @returns {Number} interquartile range