# Chi square distribution

Using the definition of moment generating function, we obtain The integral above is well-defined and finite only wheni. Thus, the moment generating function of a Chi-square random variable exists for any. The characteristic function of a Chi-square random variable is Using the definition of characteristic function, we obtain: Distribution function The distribution function of a Chi-square random variable is where the function is called lower incomplete Gamma function and is usually computed by means of specialized computer algorithms. AP stat formulas Chi-Square Distribution The distribution of the chi-square statistic is called the chi-square distribution. In this lesson, we learn to compute the chi-square statistic and find the probability associated with the statistic. The Chi-Square Statistic Suppose we conduct the following statistical experiment. We find that the standard deviation in our sample is equal to s. Given these data, we can define a statisticcalled chi-square, using the following equation: The chi-square distribution is defined by the following probability density function: Y0 is defined, so that the area under the chi-square curve is equal to one.

Similarly, the green curve shows the distribution for samples of size 5 degrees of freedom equal to 4 ; and the blue curve, for samples of size 11 degrees of freedom equal to The chi-square distribution has the following properties: The mean of the distribution is equal to the number of degrees of freedom: The variance is equal to two times the number of degrees Chi square distribution freedom: As the degrees of freedom increase, the chi-square curve approaches a normal distribution.

Cumulative Probability and the Chi-Square Distribution The chi-square distribution is constructed so that the total area under the curve is equal to 1. The area under the curve between 0 and a particular chi-square value is a cumulative probability associated with that chi-square value.

For example, in the figure below, the shaded area represents a cumulative probability associated with a chi-square statistic equal to A; that is, it is the probability that the value of a chi-square statistic will fall between 0 and A.

The easiest way to find the cumulative probability associated with a particular chi-square statistic is to use the Chi-Square Calculatora free tool provided by Stat Trek.

## Definition

Chi-Square Calculator The Chi-Square Calculator solves common statistics problems, based on the chi-square distribution. The calculator computes cumulative probabilities, based on simple inputs.

Clear instructions guide you to an accurate solution, quickly and easily. If anything is unclear, frequently-asked questions and sample problems provide straightforward explanations. The calculator is free. It can found in the Stat Trek main menu under the Stat Tools tab.

Or you can tap the button below. On average, the battery lasts 60 minutes on a single charge. The standard deviation is 4 minutes. Suppose the manufacturing department runs a quality control test.

They randomly select 7 batteries. The standard deviation of the selected batteries is 6 minutes. What would be the chi-square statistic represented by this test? Solution The standard deviation of the population is 4 minutes.

The standard deviation of the sample is 6 minutes. The number of sample observations is 7. To compute the chi-square statistic, we plug these data in the chi-square equation, as shown below. The manufacturing department ran a quality control test, using 7 randomly selected batteries.

In their test, the standard deviation was 6 minutes, which equated to a chi-square statistic of Suppose they repeated the test with a new random sample of 7 batteries.

What is the probability that the standard deviation in the new test would be greater than 6 minutes? Solution The sample size n is equal to 7. The chi-square statistic is equal to Given the degrees of freedom, we can determine the cumulative probability that the chi-square statistic will fall between 0 and any positive value.

To find the cumulative probability that a chi-square statistic falls between 0 and We say that X follows a chi-square distribution with r degrees of freedom, denoted χ 2 (r) and read "chi-square-r." There are, of course, an infinite number of possible values for r, the degrees of freedom. Statistical tables: values of the Chi-squared distribution.

What made you want to look up chi-square distribution? Please tell us where you read or heard it (including the quote, if possible). Please tell us where you read or heard it (including the quote, if possible). The area of a Chi Square distribution below 4 is the same as the area of a standard normal distribution below 2, since 4 is 2 2.

Consider the following problem: you sample two scores from a standard normal distribution, square each score, and sum the squares.

However, in a distributional modeling context (as with other probability distributions), the chi-square distribution itself can be transformed with a location parameter, μ, and a scale parameter, σ.

The following is the plot of the chi-square probability density function for 4 different values of the shape parameter. The chi-squared distribution is used in the common chi-squared tests for goodness of fit of an observed distribution to a theoretical one, the independence of two criteria of classification of qualitative data, and in confidence interval estimation for a population standard deviation of a normal distribution from a sample standard deviation.

Chi-square distribution