statistics.racket: import

.gitignore

@@ -15,3 +15,4 @@ dist/
 dist-newstyle/
 *.pdf
 *.ps
+*~

statistics.racket/shell.nix

@@ -0,0 +1,4 @@
+{ pkgs ? import <nixpkgs> {} }:
+pkgs.mkShell {
+  packages = [ pkgs.racket ];
+}

statistics.racket/statistics.rkt

@@ -0,0 +1,15 @@
+#lang typed/racket
+
+(: sum (-> (Listof Number) Number))
+(define (sum list) (foldr + 0 list))
+
+(: mean (-> (Listof Number) Number))
+(define (mean sample)
+  (/ (sum sample) (length sample)))
+
+(: standard-deviation (-> (Listof Number) Number))
+(define (standard-deviation sample)
+  (let* ((degrees-of-freedom (- (length sample) 1))
+        (sample-mean (mean sample))
+        (sum-of-squared-errors (sum (map (lambda (xi) (expt (- xi sample-mean) 2)) sample))))
+    (sqrt (/ sum-of-squared-errors degrees-of-freedom))))