Cluster samplingIn statistics, cluster sampling is a sampling plan used when mutually homogeneous yet internally heterogeneous groupings are evident in a statistical population. It is often used in marketing research. In this sampling plan, the total population is divided into these groups (known as clusters) and a simple random sample of the groups is selected. The elements in each cluster are then sampled. If all elements in each sampled cluster are sampled, then this is referred to as a "one-stage" cluster sampling plan.
Expected valueIn probability theory, the expected value (also called expectation, expectancy, expectation operator, mathematical expectation, mean, average, or first moment) is a generalization of the weighted average. Informally, the expected value is the arithmetic mean of a large number of independently selected outcomes of a random variable. The expected value of a random variable with a finite number of outcomes is a weighted average of all possible outcomes. In the case of a continuum of possible outcomes, the expectation is defined by integration.
History of multitrack recordingMultitrack recording of sound is the process in which sound and other electro-acoustic signals are captured on a recording medium such as magnetic tape, which is divided into two or more audio tracks that run parallel with each other. Because they are carried on the same medium, the tracks stay in perfect synchronization, while allowing multiple sound sources to be recorded at different times. The first system for creating stereophonic sound (using telephone technology) was demonstrated by Clément Ader in Paris in 1881.
