# Statistics chi square testing

For the test of independence, also known as the test of homogeneity, a chi-squared probability of less than or equal to 0. Assumptions[ edit ] The chi-squared test, when used with the standard approximation that a chi-squared distribution is applicable, has the following assumptions: Variants of the test have been developed for complex samples, such as where the data is weighted. Other forms can be used such as purposive sampling. The statistical null hypothesis is that the proportions of men with coronary artery disease are the same for each of the three genotypes. This indicates that you can reject the null hypothesis; the three Statistics chi square testing have significantly different proportions of men with coronary artery disease.

Graphing the results You should usually display the data used in a test of independence with a bar graph, with the values of one variable on the X-axis and the proportions of the other variable on the Y-axis.

If the variable on the Y-axis only has two values, you only need to plot one of them. In the example below, there would be no point in plotting both the percentage of men with prostate cancer and the percentage without prostate cancer; once you know what percentage have cancer, you can figure out how many didn't have cancer.

A bar graph for when the nominal variable has only two values, showing the percentage of men on different treatments who developed prostate Statistics chi square testing. If the variable on the Y-axis has more than two values, you should plot all of them.

Some people use pie charts for this, as illustrated by the data on bird landing sites from the Fisher's exact test page: A pie chart for when the nominal variable has more than two values.

The percentage of birds landing on each type of landing site is shown for herons and egrets. But as much as I like pie, I think pie charts make it difficult to see small differences in the proportions, and difficult to show confidence intervals.

In this situation, I prefer bar graphs: A bar graph for when the nominal variable has more than two values. The percentage of birds landing on each type of landing site is shown for herons gray bars and egrets black bars.

Similar tests There are several tests that use chi-square statistics. The one described here is formally known as Pearson's chi-square. It is by far the most common chi-square test, so it is usually just called the chi-square test.

The chi-square test may be used both as a test of goodness-of-fit comparing frequencies of one nominal variable to theoretical expectations and as a test of independence comparing frequencies of one nominal variable for different values of a second nominal variable.

The underlying arithmetic of the test is the same; the only difference is the way you calculate the expected values.

However, you use goodness-of-fit tests and tests of independence for quite different experimental designs and they test different null hypotheses, so I treat the chi-square test of goodness-of-fit and the chi-square test of independence as two distinct statistical tests. If the expected numbers in some classes are small, the chi-square test will give inaccurate results.

In that case, you should use Fisher's exact test. I recommend using the chi-square test only when the total sample size is greater thanand using Fisher's exact test for everything smaller than that. See the web page on small sample sizes for further discussion. If the samples are not independent, but instead are before-and-after observations on the same individuals, you should use McNemar's test.

G—test The chi-square test gives approximately the same results as the G—test. Unlike the chi-square test, G-values are additive, which means they can be used for more elaborate statistical designs. G—tests are a subclass of likelihood ratio tests, a general category of tests that have many uses for testing the fit of data to mathematical models; the more elaborate versions of likelihood ratio tests don't have equivalent tests using the Pearson chi-square statistic.

The G—test is therefore preferred by many, even for simpler designs. On the other hand, the chi-square test is more familiar to more people, and it's always a good idea to use statistics that your readers are familiar with when possible.

You may want to look at the literature in your field and see which is more commonly used. How to do the test Spreadsheet I have set up a spreadsheet that performs this test for up to 10 columns and 50 rows.

It is largely self-explanatory; you just enter you observed numbers, and the spreadsheet calculates the chi-squared test statistic, the degrees of freedom, and the P value. Web page There are many web pages that do chi-squared tests of independence, but most are limited to fairly small numbers of rows and columns.

It uses the apolipoprotein B data from above. It will work even if the sample size you end up needing is too big for a Fisher's exact test. Compute Required Sample Size. You next need to calculate the effect size parameter w.

## Chi-squared test - Wikipedia

In either case, enter made-up proportions that look like what you hope to detect. This made-up data should have proportions equal to what you expect to see, and the difference in proportions between different categories should be the minimum size that you hope to see.The Chi square test for single variance has an assumption that the population from which the sample has been is normal.

This normality assumption need not hold for chi square goodness of fit test and test for independence of attributes. The Chi-square Distribution. Before discussing the unfortunately-named "chi-square" test, it's necessary to talk about the actual chi-square attheheels.com chi-square distribution, itself, is based on a complicated mathematical formula.

Chi-Square Test for Independence. This lesson explains how to conduct a chi-square test for attheheels.com test is applied when you have two categorical variables from a single population.

It is used to determine whether there is a significant association between the two variables. Pearson's chi-squared test When testing whether observations are random variables whose distribution belongs to a given family of distributions, the "theoretical frequencies" are calculated using a distribution from that family fitted in some standard way.

Upper-tail critical values of chi-square distribution ; Degrees of freedom. A chi-squared test, also written as χ 2 test, is any statistical hypothesis test where the sampling distribution of the test statistic is a chi-squared distribution when the null hypothesis is true.

Without other qualification, 'chi-squared test' often is used as short for Pearson's chi-squared test. Using Chi-Square Statistic in Research The Chi Square statistic is commonly used for testing relationships between categorical variables.

The null hypothesis of the Chi-Square test is that no relationship exists on the categorical variables in .

Chi-Square Test of Independence