Other Notions, their Symbols and Meanings

If we have two experimental variables x and y, with mean values x and y, we should distinguish between two cases A and B. 

Case A If we have a data set of n pairs of measurements which are essentially the same, although subject to random error (the same measurement repeated), then x and y are the mean, or average, values of the sets. 

Case B If we have a data set of n different pairs of measurements (as in a calibration set, with concentration and signal) x and y are called the coordinates of the centre of gravity of the set. 

With the following sums of the squares of deviations calculated from all n pairs of measurements  

With these sums one can easily derive the variances of both variables  

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