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