Blocking

For blocking on Wikipedia, please see Wikipedia:Blocking policy.

Blocking may refer to:

• telecommunications, where the term has the following meanings:

1. The formatting of data into blocks for purposes of transmission, storage, checking, or other functions.

2. Denying access to, or use of, a facility, system, or component.

3. The failure of a telecommunications network to meet a user service demand, because of the lack of an available communications path.

• the movement and positioning of actors on the stage of a theatre

• Blocking is a term used in theatrical improvisation

• In a combat context, blocking can be a synonym for defense, thus meaning "defending from attacks by an opponent".

• In the statistical theory of the design of experiments blocking refers to arranging the experimental units in groups (blocks) which are similar to one another. This reduces sources of variability and thus leads to greater precision. When there are only 2 blocks it is known as pairing. Suppose we have invented a process which we believe makes the soles of shoes last longer, and we wish to conduct a field trial. One possible design would be to have a group of 2N volunteers, give N of them shoes with the new soles and N of them regular shoes. The assignment of new or regular soles should be by randomization. We can then let both groups use their shoes for a suitable period of time and then compare them. A better design would be to give each person one regular sole and one new sole. The choice of assigning the new sole to the right or left shoe should be by randomization. This design will be more sensitive than the first, because each person is acting as their own control and thus the control group is more closely matched to the treatment group . The theoretical basis of blocking is the following mathematical result. Given random variables, X and Y

Var(X - Y) = Var(X) + Var(Y) - 2Covar(X,Y)

The difference between the treatment and the control can thus be given minimum variance (i.e. maximum precision) by maximising the covariance (or the correlation) between X and Y.