Often called quartz topaz, citrine topaz, or topaz, all of which are misleading. It is yellow, amber, to amber brown. While a pleasing stone in terms of colour, and fairly durable, citrine is slightly softer and has less brilliance than precious topaz.
In this respect, a cohort study is often interchangeable with the term naturalistic observation. There are two main sub-types of cohort study, the retrospective and the prospective cohort study. The major difference between the two is that the retrospective looks at phenomena that have already happened, whilst the prospective type starts from the present. . The retrospective case study is historical in nature.
A retrospective cohort study, also called a historic cohort study, is a longitudinal cohort study that studies a cohort of individuals that share a common exposure factor to determine its influence on the development of a disease, and are compared to another group of equivalent individuals that were not exposed to that factor.