If we were to run an ANOVA test and we are confident that our data are approximately normally distributed, what would we also have to assume about the within-group variation?

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BlackTom AI · Accepted Answer

We start by restating the core question to set the context: when running an ANOVA with data that are approximately normally distributed, what assumption about within-group variation must hold?
Option 1: The population variance within each group (each formula) is the same as the population variance between groups.
This mixes two different concepts. ANOVA assumes equal within-group variances across groups, but it does not require the within-group variance to be equal to the between-groups variance. In fact, the between-groups variance is a function of group means and sample sizes, not a variance term to be equated with within-group variance. So this option misstates the relationship and is not the standard assumption.
Option 2: The population variance within each group (each formula) is the same.
This is the classic homogeneity of variances (homoscedasticity) assumption for one-way ANOVA. It states that the within-group variances are equal across all groups, which ensures that the F-test has its intended distribution under the null hypothesis. This aligns with the typical requirement for ANOVA when normality is assumed.
Option 3: The population variances within each group (each formula) are not the same.
This asserts heterogeneity of variances across groups. While there are robust methods or alternative tests that handle unequal variances, standard one-way ANOVA under normality explicitly assumes equal within-group variances. Therefore this option contradicts the standard assumption and would lead to potential inaccuracies if relied upon.
Option 4: The population variances within each group (each formula) is not the same as the population variance between groups.
This statement tries to distinguish within-group variance from between-groups variance but is phrased in a way that may confuse the two concepts. In ANOVA, the key assumption concerns equality of within-group variances across groups, not a required comparison between within-group and between-group variances. The wording here adds an unnecessary specificity about not equaling the between-group variance, which is not the standard assumption that ANOVA relies on.

In summary, evaluating the options against the standard ANOVA assumptions under normality, the critical requirement is that within-group variances are equal across groups, i.e., homogeneity of variances. The other choices either misstate the relationship between within- and between-group variances or assert a condition (inequality of variances) that ANOVA does not require under its typical formulation.

PSYCH 3321 SP2025 (28481) Homework 10

If we were to run an ANOVA test and we are confident that our data are approximately normally distributed, what would we also have to assume about the within-group variation?

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