Unequal sample sizes

When Unequal Sample Sizes Are and Are NOT a Problem in ANOVA

In your statistics class, your professor made a big deal about unequal sample sizes in one-way Analysis of Variance (ANOVA) for two reasons.

1. Because she was making you calculate everything by hand. Sums of squares require a different formula if sample sizes are unequal, but SPSS (and other statistical software) will automatically use the right formula.

2. Nice properties in ANOVA such as the Grand Mean being the intercept in an effect-coded regression model don’t hold when data are unbalanced. Instead of the grand mean, you need to use a weighted mean. That’s not a big deal if you’re aware of it. [Read more…] about When Unequal Sample Sizes Are and Are NOT a Problem in ANOVA

Unequal Sample Sizes

by Karen Grace-Martin 12 Comments

My next door neighbor, who is a mycologist (hey, it’s Ithaca–everyone’s a researcher here) asked me a very common statistical question–she was very concerned about her unequal sample sizes. She was doing a chi-square test and had about 11 observations in one grouping and 18 in the other.

She had already talked with a statistical consultant, who said the unequal samples were not a problem. But since she still didn’t understand why, she asked me.

As it turns out, I had just finished writing a document called “7 Statistical Issues that Researchers Shouldn’t Worry (So Much) About.” #4 is unequal sample sizes.

I wrote this because there are a number of issues that are of common concern to researchers but really aren’t a problem in all but the rarest situations. Yet they eat up a lot of mental energy.

So worry no more–get the report for free at The Analysis Factor, and read all about unequal sample sizes and more.