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.
How long does it take to finish graduate school? In most cases, a masters degree in psychology takes approximately 2 years to complete, and a doctorate takes 4-6 years (that is, 2-4 years longer than a masters degree).