Thus by 1803 there were two known rare earth elements, yttrium and cerium, although it took another 30 years for researchers to determine that other elements were contained in the two ores ceria and yttria (the similarity of the rare earth metals' chemical properties made their separation difficult).
For instance, students who pursue a Ph.D. in psychology usually take from five to seven years depending upon required coursework as well as internship and supervised residency requirements.
A doctoral degree to practice psychology requires at least 4-6 years of full-time study after completing an undergraduate degree. Coursework includes areas such as ethics, statistics, individual differences and the biological, cognitive-affective and social bases of behavior, as well as specific training in psychological assessment and therapy.