This article is about quantifying the concept of "typical value". For other uses, see Mean (disambiguation). For broader coverage of this topic, see Average. For the state of being mean or cruel, see Meanness.
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There are several kinds of mean in mathematics, especially in statistics. Each mean serves đồ sộ summarize a given group of data, often đồ sộ better understand the overall value (magnitude and sign) of a given data mix.
For a data mix, the arithmetic mean, also known as "arithmetic average", is a measure of central tendency of a finite mix of numbers: specifically, the sum of the values divided by the number of values. The arithmetic mean of a mix of numbers x_{1}, x_{2}, ..., x_{n} is typically denoted using an overhead bar, .^{[note 1]} If the data mix were based on a series of observations obtained by sampling from a statistical population, the arithmetic mean is the sample mean () đồ sộ distinguish it from the mean, or expected value, of the underlying distribution, the population mean (denoted or ^{[note 2]}).^{[1]}
Outside probability and statistics, a wide range of other notions of mean are often used in geometry and mathematical analysis; examples are given below.
Types of means[edit]
Pythagorean means[edit]
Arithmetic mean (AM)[edit]
The arithmetic mean (or simply mean) of a list of numbers, is the sum of all of the numbers divided by the number of numbers. Similarly, the mean of a sample , usually denoted by , is the sum of the sampled values divided by the number of items in the sample.
For example, the arithmetic mean of five values: 4, 36, 45, 50, 75 is:
Geometric mean (GM)[edit]
The geometric mean is an average that is useful for sets of positive numbers, that are interpreted according đồ sộ their product (as is the case with rates of growth) and not their sum (as is the case with the arithmetic mean):
- ^{[2]}
For example, the geometric mean of five values: 4, 36, 45, 50, 75 is:
Harmonic mean (HM)[edit]
The harmonic mean is an average which is useful for sets of numbers which are defined in relation đồ sộ some unit, as in the case of tốc độ (i.e., distance per unit of time):
For example, the harmonic mean of the five values: 4, 36, 45, 50, 75 is
Relationship between AM, GM, and HM[edit]
AM, GM, and HM satisfy these inequalities:
Equality holds if all the elements of the given sample are equal.
Statistical location[edit]
In descriptive statistics, the mean may be confused with the median, mode or mid-range, as any of these may incorrectly be called an "average" (more formally, a measure of central tendency). The mean of a mix of observations is the arithmetic average of the values; however, for skewed distributions, the mean is not necessarily the same as the middle value (median), or the most likely value (mode). For example, mean income is typically skewed upwards by a small number of people with very large incomes, so sánh that the majority have an income lower than thở the mean. By contrast, the median income is the level at which half the population is below and half is above. The mode income is the most likely income and favors the larger number of people with lower incomes. While the median and mode are often more intuitive measures for such skewed data, many skewed distributions are in fact best described by their mean, including the exponential and Poisson distributions.
Mean of a probability distribution[edit]
The mean of a probability distribution is the long-run arithmetic average value of a random variable having that distribution. If the random variable is denoted by , then it is also known as the expected value of (denoted ). For a discrete probability distribution, the mean is given by , where the sum is taken over all possible values of the random variable and is the probability mass function. For a continuous distribution, the mean is , where is the probability mật độ trùng lặp từ khóa function.^{[4]} In all cases, including those in which the distribution is neither discrete nor continuous, the mean is the Lebesgue integral of the random variable with respect đồ sộ its probability measure. The mean need not exist or be finite; for some probability distributions the mean is infinite (+∞ or −∞), while for others the mean is undefined.
Generalized means[edit]
Power mean[edit]
The generalized mean, also known as the power mean or Hölder mean, is an abstraction of the quadratic, arithmetic, geometric, and harmonic means. It is defined for a mix of n positive numbers x_{i} by
^{[2]}
By choosing different values for the parameter m, the following types of means are obtained:
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- maximum of
- quadratic mean
- arithmetic mean
- geometric mean
- harmonic mean
- minimum of
f-mean[edit]
This can be generalized further as the generalized f-mean
and again a suitable choice of an invertible f will give
Weighted arithmetic mean[edit]
The weighted arithmetic mean (or weighted average) is used if one wants đồ sộ combine average values from different sized samples of the same population:
- ^{[2]}
Where and are the mean and size of sample respectively. In other applications, they represent a measure for the reliability of the influence upon the mean by the respective values.
Truncated mean[edit]
Sometimes, a mix of numbers might contain outliers (i.e., data values which are much lower or much higher than thở the others). Often, outliers are erroneous data caused by artifacts. In this case, one can use a truncated mean. It involves discarding given parts of the data at the top or the bottom kết thúc, typically an equal amount at each kết thúc and then taking the arithmetic mean of the remaining data. The number of values removed is indicated as a percentage of the total number of values.
Interquartile mean[edit]
The interquartile mean is a specific example of a truncated mean. It is simply the arithmetic mean after removing the lowest and the highest quarter of values.
assuming the values have been ordered, so sánh is simply a specific example of a weighted mean for a specific mix of weights.
Mean of a function[edit]
In some circumstances, mathematicians may calculate a mean of an infinite (or even an uncountable) mix of values. This can happen when calculating the mean value of a function . Intuitively, a mean of a function can be thought of as calculating the area under a section of a curve, and then dividing by the length of that section. This can be done crudely by counting squares on graph paper, or more precisely by integration. The integration formula is written as:
In this case, care must be taken đồ sộ make sure that the integral converges. But the mean may be finite even if the function itself tends đồ sộ infinity at some points.
Mean of angles and cyclical quantities[edit]
Angles, times of day, and other cyclical quantities require modular arithmetic đồ sộ add and otherwise combine numbers. In all these situations, there will not be a unique mean. For example, the times an hour before and after midnight are equidistant đồ sộ both midnight and noon. It is also possible that no mean exists. Consider a color wheel—there is no mean đồ sộ the mix of all colors. In these situations, you must decide which mean is most useful. You can bởi this by adjusting the values before averaging, or by using a specialized approach for the mean of circular quantities.
Fréchet mean[edit]
The Fréchet mean gives a manner for determining the "center" of a mass distribution on a surface or, more generally, Riemannian manifold. Unlike many other means, the Fréchet mean is defined on a space whose elements cannot necessarily be added together or multiplied by scalars. It is sometimes also known as the Karcher mean (named after Hermann Karcher).
Triangular sets[edit]
In geometry, there are thousands of different definitions for the center of a triangle that can all be interpretted as the mean of a triangular mix of points in the plane.^{[citation needed]}
Swanson's rule[edit]
This is an approximation đồ sộ the mean for a moderately skewed distribution.^{[5]} It is used in hydrocarbon exploration and is defined as:
where P_{10}, P_{50} and P_{90} 10th, 50th and 90th percentiles of the distribution.
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Other means[edit]
See also[edit]
- Central tendency
- Median
- Mode
- Descriptive statistics
- Kurtosis
- Law of averages
- Mean value theorem
- Moment (mathematics)
- Summary statistics
- Taylor's law
Notes[edit]
- ^ Pronounced "x bar".
- ^ Greek letter μ, for "mean", pronounced /'mjuː/.
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