In many experiments, the functional relation between two or more variables describing a physical process, y = f(x1, x2, ...), is investigated by measuring the value of y for various of x1, x2, . . .It is then desired to find the parameters of a theoretical curve which best describe these points. For example, to determine the lifetime of a certain radioactive source, measurements of the count rates, N1, N2, . . ., Nn, at various times, t1, t2, . . . , t1, could be made and the data fitted to the expression
Since the count rate is subject to statistical fluctuations, the
values Ni will have uncertainties
i =
Ni and will not
all lie along
a smooth curve. What then is the best curve or equivalently, the best
values for 
and
N0 and how do we determine them? The method most
useful for this is the method of least squares.