## Probability Distributions:

When we make a measurement, we expect to get the true value. This is
known as the expected value *E*(*x*), or the
population (true) mean *μ*.
We rarely get this value, however, as there is always some degree of
random error or fluctuation in the system. The result we get will
probably not be the true value, but somewhere close to it.
If we repeat the measurements enough times, we expect that the
average will be close to the true value, with the actual results spread
around it. The distribution of a few measurements might look something
like the following histogram:

Histogram of replicate concentration determinations