Initial checkin

contrib/math/statistics.factor

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+IN: math-contrib
+USING: kernel math sequences ;
+
+: mean ( seq -- n )
+	#! arithmetic mean, sum divided by length
+    [ sum ] keep length / ;
+
+: geo-mean ( seq -- n )
+	#! geometric mean, nth root of product
+	[ product ] keep length swap nth-root ;
+
+: median ( seq -- n )
+	#! middle number if odd, avg of two middle numbers if even
+    number-sort dup length dup even? [
+		1+ 2 /i dup 1- rot [ nth ] keep swapd nth + 2 /
+	] [
+		2 / swap nth
+	] if ;
+
+: range ( seq -- n )
+	#! max - min
+	number-sort [ pop ] keep first - ;
+
+: var ( seq -- )
+	#! variance, normalize by N-1
+	dup length 1- dup 0 = [
+		0 2nip
+	] [
+	 	swap [ mean ] keep [ over - sq ] map sum nip swap /
+	] if ;
+
+: std
+	#! standard deviation, sqrt of variance
+	var sqrt ;
+