To calculate the power of a one-way ANOVA, we use the noncentral F distribution *F*(*df _{B}, df_{E}, λ*) where the noncentrality parameter is

The noncentrality parameter is also equal to *f ^{2}n* where

*f*is the effect size measure described in Effect Size for ANOVA.

**Example 1**: Find the power for the test in Example 2 of One-way ANOVA Basic Concepts.

We start by showing the results of the one-way ANOVA using Real Statistics’s data analysis tool in Figure 1.

**Figure 1 – One-way ANOVA**

Using the results in Figure 1, we now calculate the power in Figure 2.

**Figure 2 – Power of a one-way ANOVA**

We see that the power is 65.3%. We can achieve the same result using the first of the following two supplemental functions.

**Real Statistics Functions**: The following functions are provided in the Real Statistics Pack:

**ANOVA1_POWER**(*f, n, k, type, α, iter, prec*) = the power of a one-way ANOVA where *type* = 1 (default), *f* = Cohen’s effect size and *n* = the sample size. If *type* = 2 then* f* = the RMSSE effect size instead and if *type* = 0 then *f* = the noncentrality parameter.

**ANOVA1_SIZE**(*f, k, *1−*β, type, α, iter, prec*) = the minimum sample size required to obtain power of at least 1−*β* (default .80) in a of a one-way ANOVA where *type* = 1 (default), *f* = Cohen’s effect size. If *type* = 2 then *f* = the RMSSE effect size instead.

Here α = significance level (default = .05). The calculation of the infinite sum for the noncentral F distribution stops when the level of precision exceeds *prec* (default 0.000000001) or the number of terms in the infinite sum exceeds *iter* (default 1,000).

For Example 1, ANOVA1_POWER(Q11,Q9,Q10) = .652582, as expected. The same result can be achieved using the formulas

=ANOVA1_POWER(Q12,Q9,Q10,2)

=ANOVA1_POWER(Q13,Q9,Q10,0).

**Example 2**: How big a sample is required to achieve power of 80% for a one-way ANOVA with 4 groups and a Cohen’s effect size of .3?

We can use Excel’s Goal Seek capability as shown in Figure 3.

**Figure 3 – Sample size required for a one-way ANOVA using Goal Seek**

Upon clicking on the **OK** button in the Goal Seek dialog box, the results in Figure 4 are obtained.

**Figure 4 – Sample size required for a one-way ANOVA**

Rounding up we see that a sample size of 125 (cell V7) is required. Actually this sample size will be just short of 80% power (note that cell V16 is less than 80%). The actual sample size required to achieve 80% is 126.

We achieve the same result by using the supplemental formula ANOVA1_SIZE(V9,V8).

Why the NF_DIST return #NAME?

I used your example numbers here in my office365

I installed everything as your instruction, solver, real statistics pack.

Thanks.

CJ.

CJ,

It sounds like the Real Statistics software is not yet installed properly; otherwise you shouldn’t see #NAME?

When you enter =VER() into any cell what do you see?

When you press Alt-TI do you see RealStat and Solver on the list of Excel addins with check marks next to them?

Charles

What would be the distribution to prepare a lambda table for critical values for “acceptable” power levels, for example, eighty percent and up?

Eric,

For a given alpha (say .05), sample size and power (say 80%), you could use the Real Statistics tools to estimate acceptable lambda values. There would be some trial and error in this calculation (or you could use Goal Seek).

Charles