Its only advantage over the random sampling technique is simplicity. Systematic sampling is frequently used to select a specified number of records from a computer file. Stratified sampling is commonly used probability method that is superior to random sampling because it reduces sampling error. A stratum is a subset of the population that share at least one common characteristic. Examples of stratums might be males and females, or managers and non-managers.
| references | in Wikidata. Niobium, formerly columbium, is a chemical element with symbol Nb (formerly Cb) and atomic number 41. It is a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite. Its name comes from Greek mythology, specifically Niobe, who was the daughter of Tantalus, the namesake of tantalum.
21 Jun, 2012 Who discovered 0. Niobium is a soft gray metal and a chemical element represented by the symbol Nb with the atomic number 41. The free metal does not occur in nature, but it can be found in certain minerals such as columbite and pyrochlore. The largest deposits of niobium containing minerals are found in Brazil and Canada and these two countries continue to produce the majority of the element for the world. Niobium is chemically similar to the element tantalum, which meant that its discovery was the subject of much debate.