(defun evenp (number)
  (if (= 0 (% number 2))
    t
  nil))

(defun oddp (number)
  (not (evenp number)))

(defun stats:mean (list)
  (/ (apply #'+ list) (float (length list))))

(defun stats:median (list)
  (let* ((sorted (sort (copy-sequence list) #'<))
         (len (length sorted))
         (mid (floor (/ len 2))))
    (if (evenp len)
        (/ (+ (nth mid sorted) (nth (1- mid) sorted)) 2.0)
        (nth mid sorted))))

(defun stats:standard-deviation (list)
  (let ((m (stats:mean list)))
    (sqrt (/ (seq-reduce (lambda (x acc) (+ x (expt (- x m) 2))) list 0)
             (length list)))))