Power can be calculated using the *power.rmcorr* function.
This function modifies pwr.r.test from the
pwr package to use
the rmcorr degrees of freedom. It is not presently included in the
rmcorr package.

Notation: *N* is the sample size, *k* is the (average)
number of repeated measures for each individual, and
*r*_{rm} is the rmcorr effect size.

*N* = 100, *k* = 3, and
*r*_{rm} = 0.20. This design has 82% power.

```
install.packages("pwr")
require(pwr)
power.rmcorr <- function(k, N, effectsizer, sig)
{pwr.r.test(n = ((N)*(k-1))+1, r = effectsizer, sig.level = sig)}
power.rmcorr(k = 3, N = 100, effectsizer = 0.20, sig = 0.05)
#>
#> approximate correlation power calculation (arctangh transformation)
#>
#> n = 201
#> r = 0.2
#> sig.level = 0.05
#> power = 0.8156984
#> alternative = two.sided
```

See Power curves for more information.

For
G*Power
or other software, power can be calculated by substituting the rmcorr
degrees of freedom for a Pearson correlation. Instead of sample size,
use the degrees of freedom for rmcorr plus two (effective sample size).
This is because a Pearson correlation has *N* - 2 degrees of
freedom.

- Rmcorr exact degrees of freedom =
*N*x (*k*- 1) - 1 - Approximate degrees of freedom = (
*N*-1) x (*k*- 1)

Values: *N* = 100, *k* = 3 , and
*r*_{rm} = 0.20 (same as above)

rmcorr df =
100 x (3 - 1) - 1 = 200 - 1 = 199

Add two (kludge): 199 + 2 = 201

Enter *N* = 201 as the effective sample size in G*Power

G*Power will calculate the degrees of freedom as *N* - 2 = 201 -
2 = 199. Thus, using the correct degrees of freedom for rmcorr.

*Note* power is just slighty different than the same
calculation in R, which uses an arctan approximation.

Values: *N* = 100, *k* = 3, and
*r*_{rm} = 0.20 (again, same as above)

approx rmcorr df = (100 - 1) x (3 - 1) = 99 x 2 = 198

Note the
small difference between the approximate vs. exact calculation:
*one* degree of freedom

Add two (same kludge) for G*Power,
entering a sample size of *N* = 200

*Note* power is slighty different than the exact formula
above.

```
<- rmcorr(participant = Subject, measure1 = PaCO2, measure2 = pH,
my.rmc dataset = bland1995)
#> Warning in rmcorr(participant = Subject, measure1 = PaCO2, measure2 = pH, :
#> 'Subject' coerced into a factor
# Structure of rmcorr object
#str(my.rmc)
# Extract rmcorr model coefficients
<- my.rmc$model$coefficients
coef.rmc
coef.rmc#> (Intercept) Participant1 Participant2 Participant3 Participant4 Participant5
#> 7.65590848 -0.72605418 -0.02144291 0.22395850 0.24550355 0.13432752
#> Participant6 Participant7 Measure1
#> 0.20037424 -0.03394863 -0.10832305
<- coef.rmc[length(coef.rmc)] #Last value in coefficients is the slope
slope.rmc
slope.rmc#> Measure1
#> -0.108323
# Confidence intervals around all estimates
<- stats::confint(my.rmc$model)
coef.CIs <- cbind(coef.rmc, coef.CIs)
coefs.all
coefs.all#> coef.rmc 2.5 % 97.5 %
#> (Intercept) 7.65590848 7.34994594 7.96187102
#> Participant1 -0.72605418 -0.83211999 -0.61998837
#> Participant2 -0.02144291 -0.11116705 0.06828123
#> Participant3 0.22395850 0.16049969 0.28741732
#> Participant4 0.24550355 0.16448903 0.32651806
#> Participant5 0.13432752 0.05868538 0.20996967
#> Participant6 0.20037424 0.12673292 0.27401556
#> Participant7 -0.03394863 -0.17148111 0.10358385
#> Measure1 -0.10832305 -0.16883787 -0.04780822
```

We have had reports of this error when running rmcorr:

`Error in isa(Participant, "character") : could not find function "isa" `

Updating R to version 4.1.0 or later resolves this error.

Transformations can be used to make the data (errors) more normal. We
highly recommend graphing both the raw and transformed data. It may be
appropriate to only transform one measure or transform both measures.
“Consider transforming every variable in sight” (Gelman & Hill,
2017: *Data Analysis Using Regression and Multilevel/Hierarchical
Models*, ISBN: 9780521686891, p. 548):
Google
Books

In general, rmcorr is a time-independent model– it does not model
change over time. A partial exception is if time is a measure, such as
age in the raz2005 dataset.

Rmcorr fits a linear model. If the data are non-linear, we recommend trying to transform it or using multilevel modeling.

If slopes meaningfully vary by individual, we recommend using
multilevel modeling instead of rmcorr. Random effect slopes are even
more problematic for rmcorr with influential observations and/or highly
unbalanced data. This is nicely illustrated in simulations by Dr. Marta
Karas:

When
rmcorr may not be ideal

We know of three other implementations of rmcorr.