Search results
Results From The WOW.Com Content Network
The sign of the covariance of two random variables X and Y. In probability theory and statistics, covariance is a measure of the joint variability of two random variables. [1] The sign of the covariance, therefore, shows the tendency in the linear relationship between the variables.
The law of total covariance can be proved using the law of total expectation: First, (,) = [] [] [] from a simple standard identity on covariances. Then we apply the law of total expectation by conditioning on the random variable Z:
With any number of random variables in excess of 1, the variables can be stacked into a random vector whose i th element is the i th random variable. Then the variances and covariances can be placed in a covariance matrix, in which the (i, j) element is the covariance between the i th random variable and the j th one.
Throughout this article, boldfaced unsubscripted and are used to refer to random vectors, and Roman subscripted and are used to refer to scalar random variables.. If the entries in the column vector = (,, …,) are random variables, each with finite variance and expected value, then the covariance matrix is the matrix whose (,) entry is the covariance [1]: 177 ...
In this case, the covariance is the expectation of the product, and and are uncorrelated if and only if [] =. If X {\displaystyle X} and Y {\displaystyle Y} are independent , with finite second moments , then they are uncorrelated.
where is the covariance matrix of and refers to the trace of a matrix — that is, to the sum of the elements on its main diagonal (from upper left to lower right). Since the quadratic form is a scalar, so is its expectation.
The parameter belongs to the set of positive-definite matrices, which is a Riemannian manifold, not a vector space, hence the usual vector-space notions of expectation, i.e. "[^]", and estimator bias must be generalized to manifolds to make sense of the problem of covariance matrix estimation.
The proposition in probability theory known as the law of total expectation, [1] the law of iterated expectations [2] (LIE), Adam's law, [3] the tower rule, [4] and the smoothing theorem, [5] among other names, states that if is a random variable whose expected value is defined, and is any random variable on the same probability space, then