Instances of the class <a class="localLink" href="http://schema.org/Observation">Observation</a> are used to specify observations about an entity (which may or may not be an instance of a <a class="localLink" href="http://schema.org/StatisticalPopulation">StatisticalPopulation</a>), at a particular time. The principal properties of an <a class="localLink" href="http://schema.org/Observation">Observation</a> are <a class="localLink" href="http://schema.org/observedNode">observedNode</a>, <a class="localLink" href="http://schema.org/measuredProperty">measuredProperty</a>, <a class="localLink" href="http://schema.org/measuredValue">measuredValue</a> (or <a class="localLink" href="http://schema.org/median">median</a>, etc.) and <a class="localLink" href="http://schema.org/observationDate">observationDate</a> (<a class="localLink" href="http://schema.org/measuredProperty">measuredProperty</a> properties can, but need not always, be W3C RDF Data Cube "measure properties", as in the <a href="https://www.w3.org/TR/vocab-data-cube/#dsd-example">lifeExpectancy example</a>). See also <a class="localLink" href="http://schema.org/StatisticalPopulation">StatisticalPopulation</a>, and the <a href="/docs/data-and-datasets.html">data and datasets</a> overview for more details.