# Variance And Standard Deviation In Statistics Pdf

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- STATISTICS AND STANDARD DEVIATION Statistics and Standard Deviation
- 2.8 – Expected Value, Variance, Standard Deviation
- Calculating Variance and Standard Deviation

## STATISTICS AND STANDARD DEVIATION Statistics and Standard Deviation

Previous: 2. Next: 2. Analogous to the discrete case, we can define the expected value, variance, and standard deviation of a continuous random variable. These quantities have the same interpretation as in the discrete setting.

The expectation of a random variable is a measure of the centre of the distribution, its mean value. The variance and standard deviation are measures of the horizontal spread or dispersion of the random variable. The following animation encapsulates the concepts of the CDF, PDF, expected value, and standard deviation of a normal random variable.

When viewing the animation, it may help to remember that. The random variable X is given by the following PDF.

Check that this is a valid PDF and calculate the standard deviation of X. To verify that f x is a valid PDF, we must check that it is everywhere nonnegative and that it integrates to 1. To calculate the standard deviation of X , we must first find its variance. Calculating the variance of X requires its expected value:. There is an alternative formula for the variance of a random variable that is less tedious than the above definition.

The derivation of this formula is a simple exercise and has been relegated to the exercises. We should note that a completely analogous formula holds for the variance of a discrete random variable, with the integral signs replaced by sums.

We can use this alternate formula for variance to find the standard deviation of the random variable X defined above. In the exercises, you will compute the expectations, variances and standard deviations of many of the random variables we have introduced in this chapter, as well as those of many new ones.

You can download a PDF version of both lessons and additional exercises here. What is the most difficult concept to understand in probability? View Results. Anyone has the right to use this work for any purpose, without any conditions, unless such conditions are required by law.

If you are having trouble viewing this website, please see the Technical Requirements page. Please visit our contact page for questions and comments. Skip to content. Solution Part 1 To verify that f x is a valid PDF, we must check that it is everywhere nonnegative and that it integrates to 1. To check that f x has unit area under its graph, we calculate So f x is indeed a valid PDF.

Part 2 To calculate the standard deviation of X , we must first find its variance. Calculating the variance of X requires its expected value: Using this value, we compute the variance of X as follows Therefore, the standard deviation of X is An Alternative Formula for Variance There is an alternative formula for the variance of a random variable that is less tedious than the above definition. Alternate Formula for the Variance of a Continuous Random Variable The variance of a continuous random variable X with PDF f x is the number given by The derivation of this formula is a simple exercise and has been relegated to the exercises.

Simple Example Revisited We can use this alternate formula for variance to find the standard deviation of the random variable X defined above.

Search for:. How to calculate a PDF when give a cumulative distribution function. The difference between discrete and continuous random variables. In MATH , there are no difficult topics on probability. Technical Requirements If you are having trouble viewing this website, please see the Technical Requirements page. Acronyms Throughout this website, the following acronyms are used. Contact Please visit our contact page for questions and comments. Proudly powered by WordPress. Spam prevention powered by Akismet.

The variance of a continuous random variable X with PDF f x is the number given by.

## 2.8 – Expected Value, Variance, Standard Deviation

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## Calculating Variance and Standard Deviation

You can draw a histogram of the pdf and find the mean, variance, and standard deviation of it. For a general discrete probability distribution, you can find the mean, the variance, and the standard deviation for a pdf using the general formulas. These formulas are useful, but if you know the type of distribution, like Binomial, then you can find the mean and standard deviation using easier formulas. They are derived from the general formulas.

In probability theory , a normal or Gaussian or Gauss or Laplace—Gauss distribution is a type of continuous probability distribution for a real-valued random variable. The general form of its probability density function is. Normal distributions are important in statistics and are often used in the natural and social sciences to represent real-valued random variables whose distributions are not known. It states that, under some conditions, the average of many samples observations of a random variable with finite mean and variance is itself a random variable—whose distribution converges to a normal distribution as the number of samples increases.

#### Tabulating results

This means that over the long term of doing an experiment over and over, you would expect this average. If you repeat this experiment toss three fair coins a large number of times, the expected value of X is the number of heads you expect to get for each three tosses on average. It represents the mean of a population. A men's soccer team plays soccer zero, one, or two days a week. The probability that they play zero days is.

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