movement in bed), but they are generally considered inferior to the gold standard of polysomnography (which uses measurement of brain waves to quantify the amount of time a person spends in each stage of sleep). Often the gold standard is more difficult or expensive to perform, and the cheaper method is used even though it might have greater error.</p>
<p>When we think about what makes a good measurement, we usually distinguish two different aspects of a good measurement: it should be <em>reliable</em>, and