Chi square goodness of fit test specifically tells how well a categorical (nominal or ordinal) sample distribution fit into a hypothetical distribution. In other words, it is the test used to check if the sample data is consistent with a hypothesized distribution of the population. The goodness of fit test is used to determine the observed sample distribution matches or fits the expected values; hence we use the term goodness of fit.

The goodness of fit is a non-parametric test because it does not rely on estimates of a population parameter like mean or variance to make an inference on the characteristics of the population.

When to use Chi Square Goodness of Fit?

A chi-square goodness-of-fit test can be conducted when there is one categorical variable with more than two levels. If there are exactly two categories, then a one proportion z test may be conducted.

Reference.

The chi-square goodness-of-fit test requires 2 assumptions^2,3:

1. independent observations;

2. for 2 categories, each expected frequency EiEi must be at least 5.

For 3+ categories, each EiEi must be at least 1 and no more than 20% of all EiEi may be smaller than 5.

Reference

Structure of a Chi Square Goodness of Fit Test

The Goodness of fit tests are structured in cells; therefore, the observed frequency goes in each cell. Furthermore, the distribution you are trying to match would have a theoretical frequency. Then, the Chi square is summed across all cells.

Use the data values structured into cells and explicitly requires a calculated chi-square test statistic. The unknown distribution is tested, and likewise, the Degrees of Freedom vary according to the distribution.

GOF Distribution  | Degrees of Freedom

Normal | # cells – 3

Poisson | # cells – 2

Binomial | # cells – 2

Uniform | # cells – 1

Steps to perform Chi Square goodness of fit

Step1:

Firstly, define the null hypothesis and alternative hypothesis

• Null hypothesis (H₀): There is no difference between the observed value and the expected value
• Alternative hypothesis (H₁): There  is a significant difference between the observed value and the expected value

Step 2:

Secondly, specify the level of significance

Step 3:

Thirdly, compute the χ2 statistic

• O is the observed value
• E is the expected value

Step 4:

Fourthly, calculate the degree of freedom:

The degrees of freedom in chi square test depend on the sample distribution

Step 5:

Then, find the critical value, based on degrees of freedom

Step 6:

Finally, draw the statistical conclusion:

If the test statistic value is greater than the critical value, reject the null hypothesis, and hence we can conclude that there is a significant difference between the observed value and the expected value.

Chi Square goodness of fit test Example 1: Did the Distribution Change?

Ten years ago, US airlines categorized the priority customers (those who completed 10,000 miles traveled in a year) based specifically on age

Similarly, in 2020, 500 priority passengers are sampled, and below are the results

At a 95% confidence level, would you conclude that the population distribution of priority customers changed in the last 10 years?

• Null hypothesis (H₀): The sample data meet the expected distribution.
• Alternative hypothesis (H₁): The sample data does not meet the expected distribution.

Level of significance: α=0.05

Degrees of freedom = number of categories (n)= 4

n-1 =3

Chi-square critical value for 3 degrees of freedom =7.815

The test statistic value is greater than the critical value; hence, we can reject the null hypothesis.

So, we can conclude that the priority customers in 2020 are different than those expected based on the 2010 population.

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Chi Square Normality Test

Many statistical techniques (regression, ANOVA, t-tests, etc.) rely on the assumption that data is normally distributed. Hence, the Chi-square goodness of fit test is one of the good options to check whether the data follows a normal distribution.

Furthermore, the Chi-square goodness of fit test is an alternative to the Anderson-Darling test, Kolmogorov-Smirnov (K-S) test, and Shapiro-Wilk test to test the normality.

Similar to other statistical hypothesis tests, the Chi-square goodness of fit test also needs to compute the test statistics and find the critical value for a given degree of freedom and confidence level. If the Chi-Square value is greater than the critical value, reject the null hypothesis.

How to conduct Chi Square goodness fit normality test

• Determine the null hypothesis, i.e. data is sampled from a normal distribution and alternative hypothesis.
• Compute the sample mean as well as the standard deviation.
• Then, define the confidence level
• Bin the data: Determine non-overlapping bins, and then count values in each bin
• Furthermore, find the cumulative probability for each category endpoint.
• After that, compute the probability that a randomly selected value would go in each category
• Find expected observations for each bin, which is the product of the probability of observation would fall in the bin compared to the sample size (n)
• Likewise, compute the chi-square statistic. χ2 = Σ [ ( Observed frequency – Exp frequency) 2 / Exp frequency]
• Find the degrees of freedom based specifically on the number of categories or bins. The degrees of freedom for the Chi-square goodness of fit test is always the number of categories-1-2(two estimated parameters, mean and standard deviation) = k-3 (note: valid if mean and standard deviation are not given)
• Find the critical value, based on degrees of freedom.
• Finally, draw the statistical conclusion: If the test statistic value is greater than the critical value, reject the null hypothesis.

Chi Square normality test example

Example: 28 students’ weights (in kgs) are collected in ABC school. Test whether the data is normally distributed given that the confidence level is 95%.

• H₀: Students’ weights in an ABC school follow a normal distribution
• H₁: Students’ weights in an ABC school do not follow a normal distribution

Firstly, compute the sample mean and standard deviation.

• Select all the data and enter” =Average(B2:H5)” for mean and “ Stdev(B2:H5)” for standard deviation
• Confidence level = 95%

Secondly, bin or categories the data and count the values

Thirdly, find the cumulative probability for each category endpoint.

• In cell H11 type NORMDIST (20, L2, L4, TRUE),
• Similarly H12 =  NORMDIST (25, L2, L4, TRUE),
• H13 =  NORMDIST (30, L2, L4, TRUE),
• H14=  NORMDIST (35, L2, L4, TRUE),
• H15 =  NORMDIST (∞, L2, L4, TRUE),

Fourthly, compute the probability that a randomly selected value would go in each category

• For I 11 cell, ie less than 20 copy the same value ie 0.118431892
• In cell I 12, i.e. values between 25 to 20= H12-H11 = 0.350912244-0.118431892 = 0.232480352
• Similarly for I13 = H13-H12=0.310801442
• I14 = H14-H13 = 0.226512236
• I15 = H15-H14 = 0.111773954

Then, find expected observations in each bin, which is the product of the probability of observation would fall in the bin to the sample size (n) =28

• J11 = I11*$G$16 = 3.316092984
• Similarly, J12=I12*$G$16=6.509449848
• J13=I13*$G$16=8.702440383
• J14=I14*$G$16=6.342346084
• J15= I15*$G$16=3.129670701

Finally, find the chi-square statistic. χ² = Σ [ ( Observed frequency – Exp frequency) ² / Exp frequency]

• K11 = (G11-J11)²/J11 = 0.522331777
• Similarly, K12 =  (G12 – J12)² / J12 = 1.871731198
• K13=(G13-J13)²/J13=0.056699325
• K14=(G14-J14)²/J14 = 0.865071869
• K15=(G15-J15)²/J15 = 0.242029645
• Then, find the sum of Chi-Square values = K16= sum(K11:K15) =3.55786

• Following that logic, the Degrees of freedom = No of categories -3 =5-3 =2
• Consequently, the Critical value =5.991

Conclusion: The chi-square value of 3.557 is less than the critical value of 5.991. In hypothesis testing, a critical value is a point on the test distribution that is compared to the test statistic to determine whether to reject the null hypothesis. if the chi-square calculated was smaller than the critical value, then the data did fit the model, therefore, failed to reject the null hypothesis. So, students’ weights in ABC school follow a normal distribution.

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Chi Square Normality Testing

• https://www.statisticshowto.datasciencecentral.com/chi-square-test-normality/