The χ2 goodness-of-fit test is used to test the reasonableness of assuming sample data observations come from, or can be modeled by, particular probability distributions. One might want to see if observed arrival data can be modeled by a Poisson distribution with the sample's mean, or if observed heights can be modeled by a Normal distribution with the sample's mean and standard deviation, or if failures in a production process can be modeled by a Binomial distribution with the sample's percent defective.