Blocking is a technique (procedure) that is used in biological / agricultural field experiments to arrange experimental units in groups or blocks that are similar to one another. It is an attemp to improve comparisons of treatments.
Blocking will reduce the source of variability and increase precision (and may be accuracy)
The theoritical basis of blocking is represented b the following mathematical results. Given X and Y are random variables
VAR(X-Y)=VAR(X)+VAR (Y)-2 COV(X,Y)
Phendukani Hlatshwayo (M.Sc Agronomy, Candidate from South Africa)
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It is a qualitative variable.
When referring to statistics, variable means that numbers change and can go up and down. If something is constant, numbers remain unchanged.
In statistics, letter such as; a,..x..,z, is a variable used to represent an unknown value.
It can be though more often it is a variable on the interval scale (when looking for trends over time).
in statistics, summation denoted by upper case sigma, is used to find the sum of a series of observation in a particular variable.