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.
Thorium is found in nature in the form of the oxide thorite, ThO2, which forms crystals as thorianite . It appears in the form of thorium phosphate in monazite sand. Thorium is found in the mineral aeschynite and euxenite. Thorium has been used in the manufacture of mantles for gas lanterns. The fabric of the mantles is saturated with thorium nitrate and cerium nitrate. The burning of the mantles leaves a residue of thorium and cerium dioxides.