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In statistics, the standard deviation (SD, also represented by the Greek letter sigma σ or the Latin letter s) is a measure that is used to quantify the amount of variation or dispersion of a set of data values.

A low standard deviation indicates that the data points tend to be close to the mean (also called the expected value) of the set, while a high standard deviation indicates that the data points are spread out over a wider range of values.

Statistics is a branch of mathematics dealing with the collection, organization, analysis, interpretation and presentation of data.

In applying statistics to, for example, a scientific, industrial, or social problem, it is conventional to begin with a statistical population or a statistical model process to be studied.

Populations can be diverse topics such as "all people living in a country" or "every atom composing a crystal".

Statistics deals with all aspects of data including the planning of data collection in terms of the design of surveys and experiments.

The median is the value separating the higher half from the lower half of a data sample (a population or a probability distribution).

For a data set, it may be thought of as the "middle" value.

For example, in the data set, the median is 6, the fourth largest, and also the fourth smallest, number in the sample.

For a continuous probability distribution, the median is the value such that a number is equally likely to fall above or below it.

In mathematics, mean has several different definitions depending on the context.

In probability and statistics, population mean and expected value are used synonymously to refer to one measure of the central tendency either of a probability distribution or of the random variable characterized by that distribution.

In the case of a discrete probability distribution of a random variable X, the mean is equal to the sum over every possible value weighted by the probability of that value; that is, it is computed by taking the product of each possible value x of X and its probability p(x), and then adding all these products together, giving.

An analogous formula applies to the case of a continuous probability distribution.

Not every probability distribution has a defined mean; see the Cauchy distribution for an example.

Moreover, for some distributions the mean is infinite.

The ampere (symbol: A), often shortened to "amp", is the base unit of electric current in the International System of Units (SI).

It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.