Physical scientists use a lot of common sense and only a little statistics in dealing with errors. Only in special areas (such as the work of Standards Laboratories) are errors relentlessly pursued with full rigour. Usually, it is enough to be satisfied that they are negligible or insignificant for the purposes at hand. As with safety regulations, one tries to ensure a large ‘margin of error’ wherever possible. This contrasts with the state of affairs in biology and in social sciences. These deal in quantities, such as the frequency of the human heartbeat, so widely variable that errors must be treated with caution and proper statistical methodology. But even common sense needs a little mathematical background, such as is given here.

First, we may distinguish between random and systematic errors. A systematic error is one which is consistent from one measurement to the next. It might arise from inaccurate adjustment of instruments, a faulty calibration, the ineptitude of the scientist himself, or simply from a failure to recognise some influence upon the data which was not the object of the experiment. We can try to identify such errors and either eliminate them or add corrections to allow for them, whereupon they no longer concern us. It is not usual to include systematic errors in an error estimate since, if we can identify them, they can be removed!