Let xij be the ith independently drawn observation (i=1,...,N) on the jth random variable (j=1,...,K). These observations can be arranged into N column vectors, each with K entries, with the K ×1 column vector giving the ith observations of all variables being denoted xi (i=1,...,N). I have been reading the following link http://en.wikipedia.org/wiki/Empirical_mean to learn the Sample mean and sample covariance. But I am failing to understand the above part. Could someone please explain it to me in a easiest way?