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**STATISTICS Random Variables and Distribution Functions**

Professor Ke-Sheng Cheng Department of Bioenvironmental Systems Engineering National Taiwan University

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**Definition of random variable (RV)**

For a given probability space ( ,A, P[]), a random variable, denoted by X or X(), is a function with domain and counterdomain the real line. The function X() must be such that the set Ar, denoted by , belongs to A for every real number r. Unlike the probability which is defined on the event space, a random variable is defined on the sample space. 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**is defined whereas is not defined.**

Random experiment Sample space Event space Probability space is defined whereas is not defined. 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Cumulative distribution function (CDF)**

The cumulative distribution function of a random variable X, denoted by , is defined to be 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Consider the experiment of tossing two fair coins**

Consider the experiment of tossing two fair coins. Let random variable X denote the number of heads. CDF of X is 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Indicator function or indicator variable**

Let be any space with points and A any subset of . The indicator function of A, denoted by , is the function with domain and counterdomain equal to the set consisting of the two real numbers 0 and 1 defined by 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Discrete random variables**

A random variable X will be defined to be discrete if the range of X is countable. If X is a discrete random variable with values then the function denoted by and defined by is defined to be the discrete density function of X. 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Continuous random variables**

A random variable X will be defined to be continuous if there exists a function such that for every real number x. The function is called the probability density function of X. 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Properties of a CDF is continuous from the right, i.e. 3/27/2017**

Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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Properties of a PDF 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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Example 1 Determine which of the following are valid distribution functions: 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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Example 2 Determine the real constant a, for arbitrary real constants m and 0 < b, such that is a valid density function. 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Function is symmetric about m.**

3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Characterizing random variables**

Cumulative distribution function Probability density function Expectation (expected value) Variance Moments Quantile Median Mode 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Expectation of a random variable**

The expectation (or mean, expected value) of X, denoted by or E(X) , is defined by: 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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Rules for expectation Let X and Xi be random variables and c be any real constant. 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Variance of a random variable**

3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**is called the standard deviation of X.**

Variance characterizes the dispersion of data with respect to the mean. Thus, shifting a density function does not change its variance. 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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Rules for variance 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Events A and B are defined to be independent if and only if**

Two random variables are said to be independent if knowledge of the value assumed by one gives no clue to the value assumed by the other. Events A and B are defined to be independent if and only if 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Moments and central moments of a random variable**

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**Properties of moments 3/27/2017**

Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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Quantile The qth quantile of a random variable X, denoted by , is defined as the smallest number satisfying Discrete Uniform 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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Median and mode The median of a random variable is the 0.5th quantile, or The mode of a random variable X is defined as the value u at which is the maximum of 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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Note: For a positively skewed distribution, the mean will always be the highest estimate of central tendency and the mode will always be the lowest estimate of central tendency (assuming that the distribution has only one mode). For negatively skewed distributions, the mean will always be the lowest estimate of central tendency and the mode will be the highest estimate of central tendency. In any skewed distribution (i.e., positive or negative) the median will always fall in-between the mean and the mode. 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Moment generating function**

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3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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Usage of MGF MGF can be used to express moments in terms of PDF parameters and such expressions can again be used to express mean, variance, coefficient of skewness, etc. in terms of PDF parameters. Random variables of the same MGF are associated with the same type of probability distribution. 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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The moment generating function of a sum of independent random variables is the product of the moment generating functions of individual random variables. 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Expected value of a function of a random variable**

3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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If Y=g(X) 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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Y Y=g(X) y X x1 x2 x3 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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Theorem 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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**Chebyshev Inequality 3/27/2017**

Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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The Chebyshev inequality gives a bound, which does not depend on the distribution of X, for the probability of particular events described in terms of a random variable and its mean and variance. 3/27/2017 Laboratory for Remote Sensing Hydrology and Spatial Modeling, Dept of Bioenvironmental Systems Engineering, National Taiwan Univ.

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