Compute effect size indices for standardized difference between two normal
multivariate distributions or between one multivariate distribution and a
defined point. This is the standardized effect size for Hotelling's \(T^2\)
test (e.g., `DescTools::HotellingsT2Test()`

). *D* is computed as:

$$D = \sqrt{(\bar{X}_1-\bar{X}_2-\mu)^T \Sigma_p^{-1} (\bar{X}_1-\bar{X}_2-\mu)}$$

Where \(\bar{X}_i\) are the column means, \(\Sigma_p\) is the *pooled*
covariance matrix, and \(\mu\) is a vector of the null differences for each
variable. When there is only one variate, this formula reduces to Cohen's
*d*.

## Usage

```
mahalanobis_d(
x,
y = NULL,
data = NULL,
pooled_cov = TRUE,
mu = 0,
ci = 0.95,
alternative = "greater",
verbose = TRUE,
...
)
```

## Arguments

- x, y
A data frame or matrix. Any incomplete observations (with

`NA`

values) are dropped.`x`

can also be a formula (see details) in which case`y`

is ignored.- data
An optional data frame containing the variables.

- pooled_cov
Should equal covariance be assumed? Currently only

`pooled_cov = TRUE`

is supported.- mu
A named list/vector of the true difference in means for each variable. Can also be a vector of length 1, which will be recycled.

- ci
Confidence Interval (CI) level

- alternative
a character string specifying the alternative hypothesis; Controls the type of CI returned:

`"two.sided"`

(default, two-sided CI),`"greater"`

or`"less"`

(one-sided CI). Partial matching is allowed (e.g.,`"g"`

,`"l"`

,`"two"`

...). See*One-Sided CIs*in effectsize_CIs.- verbose
Toggle warnings and messages on or off.

- ...
Not used.

## Details

To specify a `x`

as a formula:

Two sample case:

`DV1 + DV2 ~ group`

or`cbind(DV1, DV2) ~ group`

One sample case:

`DV1 + DV2 ~ 1`

or`cbind(DV1, DV2) ~ 1`

## Confidence (Compatibility) Intervals (CIs)

Unless stated otherwise, confidence (compatibility) intervals (CIs) are
estimated using the noncentrality parameter method (also called the "pivot
method"). This method finds the noncentrality parameter ("*ncp*") of a
noncentral *t*, *F*, or \(\chi^2\) distribution that places the observed
*t*, *F*, or \(\chi^2\) test statistic at the desired probability point of
the distribution. For example, if the observed *t* statistic is 2.0, with 50
degrees of freedom, for which cumulative noncentral *t* distribution is *t* =
2.0 the .025 quantile (answer: the noncentral *t* distribution with *ncp* =
.04)? After estimating these confidence bounds on the *ncp*, they are
converted into the effect size metric to obtain a confidence interval for the
effect size (Steiger, 2004).

For additional details on estimation and troubleshooting, see effectsize_CIs.

## CIs and Significance Tests

"Confidence intervals on measures of effect size convey all the information
in a hypothesis test, and more." (Steiger, 2004). Confidence (compatibility)
intervals and p values are complementary summaries of parameter uncertainty
given the observed data. A dichotomous hypothesis test could be performed
with either a CI or a p value. The 100 (1 - \(\alpha\))% confidence
interval contains all of the parameter values for which *p* > \(\alpha\)
for the current data and model. For example, a 95% confidence interval
contains all of the values for which p > .05.

Note that a confidence interval including 0 *does not* indicate that the null
(no effect) is true. Rather, it suggests that the observed data together with
the model and its assumptions combined do not provided clear evidence against
a parameter value of 0 (same as with any other value in the interval), with
the level of this evidence defined by the chosen \(\alpha\) level (Rafi &
Greenland, 2020; Schweder & Hjort, 2016; Xie & Singh, 2013). To infer no
effect, additional judgments about what parameter values are "close enough"
to 0 to be negligible are needed ("equivalence testing"; Bauer & Kiesser,
1996).

## Plotting with `see`

The `see`

package contains relevant plotting functions. See the plotting vignette in the `see`

package.

## References

Del Giudice, M. (2017). Heterogeneity coefficients for Mahalanobis' D as a multivariate effect size. Multivariate Behavioral Research, 52(2), 216-221.

Mahalanobis, P. C. (1936). On the generalized distance in statistics. National Institute of Science of India.

Reiser, B. (2001). Confidence intervals for the Mahalanobis distance. Communications in Statistics-Simulation and Computation, 30(1), 37-45.

## See also

`stats::mahalanobis()`

, `cov_pooled()`

Other standardized differences:
`cohens_d()`

,
`means_ratio()`

,
`p_superiority()`

,
`rank_biserial()`

,
`repeated_measures_d()`

## Examples

```
## Two samples --------------
mtcars_am0 <- subset(mtcars, am == 0,
select = c(mpg, hp, cyl)
)
mtcars_am1 <- subset(mtcars, am == 1,
select = c(mpg, hp, cyl)
)
mahalanobis_d(mtcars_am0, mtcars_am1)
#> Mahalanobis' D | 95% CI
#> ----------------------------
#> 2.14 | [1.22, Inf]
#>
#> - One-sided CIs: upper bound fixed at [Inf].
# Or
mahalanobis_d(mpg + hp + cyl ~ am, data = mtcars)
#> Mahalanobis' D | 95% CI
#> ----------------------------
#> 2.14 | [1.22, Inf]
#>
#> - One-sided CIs: upper bound fixed at [Inf].
mahalanobis_d(mpg + hp + cyl ~ am, data = mtcars, alternative = "two.sided")
#> Mahalanobis' D | 95% CI
#> -----------------------------
#> 2.14 | [1.07, 2.90]
# Different mu:
mahalanobis_d(mpg + hp + cyl ~ am,
data = mtcars,
mu = c(mpg = -4, hp = 15, cyl = 0)
)
#> Mahalanobis' D | 95% CI
#> ----------------------------
#> 1.90 | [1.00, Inf]
#>
#> - Deviation from a difference of 15.5242.
#> - One-sided CIs: upper bound fixed at [Inf].
# D is a multivariate d, so when only 1 variate is provided:
mahalanobis_d(hp ~ am, data = mtcars)
#> Mahalanobis' D | 95% CI
#> ----------------------------
#> 0.49 | [0.00, Inf]
#>
#> - One-sided CIs: upper bound fixed at [Inf].
cohens_d(hp ~ am, data = mtcars)
#> Cohen's d | 95% CI
#> -------------------------
#> 0.49 | [-0.23, 1.21]
#>
#> - Estimated using pooled SD.
# One sample ---------------------------
mahalanobis_d(mtcars[, c("mpg", "hp", "cyl")])
#> Mahalanobis' D | 95% CI
#> ----------------------------
#> 12.59 | [9.49, Inf]
#>
#> - One-sided CIs: upper bound fixed at [Inf].
# Or
mahalanobis_d(mpg + hp + cyl ~ 1,
data = mtcars,
mu = c(mpg = 15, hp = 5, cyl = 3)
)
#> Mahalanobis' D | 95% CI
#> ----------------------------
#> 5.31 | [3.97, Inf]
#>
#> - Deviation from a difference of 16.0935.
#> - One-sided CIs: upper bound fixed at [Inf].
```