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.
Anyone working in psychology with a masterâs degree is usually supervised by someone with a doctoral degree. In most states, the independent practice of psychology requires a doctoral degree and a state license. Most doctoral degrees take five to seven years to complete.