Quantile support

lib/stats.js

@@ -169,26 +169,40 @@
 			}
 		},
 
-		// Finds the first quartile of the numbers.
+		// Finds a quantile with the given ratio
 		//
-		// Returns the first quartile.
-		q1: function() {
+		// Returns that quantile.
+		quantile: function(ratio) {
 			// Sort the numbers
 			var nums = this.clone().sort();
+                        var arr = nums.arr;
+
+                        // 0 and 1 are bounds; clamp to them
+                        if (ratio <= 0.0) return arr[0];
+                        if (ratio >= 1.0) return arr[arr.length - 1];
+
+                        // Determine which index represents the quantile,
+                        // averaging if it falls on a boundary
+                        var idx = (ratio * arr.length) - 1;
+                        if (idx === Math.floor(idx)) {
+                              return 0.5 * (arr[idx] + arr[idx + 1]);
+                        } else {
+                              return arr[Math.ceil(idx)];
+                        }
+		},
 
-			// The first quartile is the median of the lower half of the numbers
-			return nums.slice(0, Math.floor(nums.size() / 2)).median();
+		// Finds the first quartile of the numbers.
+		//
+		// Returns the first quartile.
+		q1: function() {
+                        return this.quantile(0.25);
 		},
 
 		// Finds the third quartile of the numbers.
 		//
 		// Returns the third quartile.
 		q3: function() {
-			// Sort the numbers
-			var nums = this.clone().sort();
-
-			// The third quartile is the median of the upper half of the numbers
-			return nums.slice(Math.ceil(nums.size() / 2)).median();
+                        return this.quantile(0.75);
 		},
 
 		// Finds the interquartile range of the data set.

package.json

@@ -5,6 +5,9 @@
 	"author": "Angus Gibbs (http://angusgibbs.com)",
 	"homepage": "http://github.com/angusgibbs/statsjs",
 	"keywords": ["math", "stats", "statistics", "average", "mean", "min", "max", "standard", "deviation", "regression", "quartile", "median", "outliers", "factorial", "probability", "permutation", "combination", "binomial", "geometric", "normal", "z-test"],
+        "scripts": {
+                "test": "mocha test/*.js"
+        },
 	"repository": {
 		"type": "git",
 		"url": "git://github.com/angusgibbs/statsjs.git"
@@ -23,4 +26,4 @@
 		"mocha": "1.4.x",
 		"expect.js": "git://github.com/angusgibbs/expect.js.git"
 	}
-}
+}

test/test.js

@@ -30,6 +30,16 @@ describe('#median()', function() {
 	});
 });
 
+describe('#quantile()', function () {
+	it('should return the proper median and quantiles', function() {
+		expect(stats([3, 4, 4, 5, 6, 8, 9]).quantile(0)).to.equal(3);
+		expect(stats([3, 4, 4, 5, 6, 8, 9]).quantile(0.25)).to.equal(4);
+		expect(stats([3, 4, 4, 5, 6, 8, 9]).quantile(0.50)).to.equal(5);
+		expect(stats([3, 4, 4, 5, 6, 8, 9]).quantile(0.75)).to.equal(8);
+		expect(stats([3, 4, 4, 5, 6, 8, 9]).quantile(1)).to.equal(9);
+	});
+});
+
 describe('#q1()', function() {
 	it('should return the first quartile of an odd length array', function() {
 		expect(stats([12, 19, 4, 1, 2, 5, 8]).q1()).to.equal(2);