`R/bimodality.R`

`mt_check_bimodality.Rd`

Assess bimodality of the distribution of mouse-tracking measures using the
bimodality coefficient and Hartigan's dip statistic (see Details). If
bimodality should be assessed separately for different conditions, the
corresponding variables can be specified under `grouping_variables`

.

```
mt_check_bimodality(
data,
use = "measures",
use_variables = NULL,
methods = c("BC", "HDS"),
B = 2000,
grouping_variables = NULL,
...
)
```

- data
a mousetrap data object created using one of the mt_import functions (see mt_example for details).

- use
a character string specifying which data should be used. By default, points to the

`measures`

data.frame created using mt_measures.- use_variables
a vector specifying for which mouse-tracking measures bimodality should be assessed.

- methods
a character string (or vector) specifying which methods should be used for assessing bimodality (see Details).

- B
an integer specifying the number of replicates used in the Monte Carlo test (only relevant if "HDS_sim" is included in methods, see Details).

- grouping_variables
a character string (or vector) specifying one or more variables in

`data[["data"]]`

. If specified, bimodality will be assessed separately for each level of the variable. If unspecified (the default), bimodality is checked across all trials.- ...
additional arguments passed on to mt_reshape (such as

`subset`

).

A list of several data.frames. Each data.frame contains the value returned by the respective method for assessing bimodality (see Details) - separately per condition (specified in the row dimension) and measure (specified in the column dimension).

Different methods have been suggested for assessing the bimodality of mouse-tracking measure distributions, each of which has advantages and disadvantages (see Freeman & Dale, 2013).

Hehman et al. (2015) focus on two specific methods (bimodality coefficient and Hartigan's dip statistic) which are implemented here.

If `methods`

include `BC`

, the bimodality coefficient is calculated
using the bimodality_coefficient function in this package. According
to Freeman and Ambady (2010), a distribution is considered bimodal if
`BC > 0.555`

.

Note that MouseTracker (Freeman & Ambady, 2010) standardizes variables within each subject before computing the BC. This is also possible here using mt_standardize (see Examples).

If `methods`

include `HDS`

, Hartigan's dip statistic is calculated
using the dip.test function of the `diptest`

package.
The corresponding p value (computed via linear interpolation) is returned.

If `methods`

include `HDS_sim`

, Hartigan's dip statistic is
calculated using the dip.test function with the additional
argument `simulate.p.values=TRUE`

. In this case, the p value is computed
from a Monte Carlo simulation of a uniform distribution with B (default:
2000) replicates.

Freeman, J. B., & Ambady, N. (2010). MouseTracker: Software for studying
real-time mental processing using a computer mouse-tracking method.
*Behavior Research Methods, 42*(1), 226-241.

Freeman, J. B., & Dale, R. (2013).
Assessing bimodality to detect the presence of a dual cognitive process.
*Behavior Research Methods, 45*(1), 83-97.

Hehman, E., Stolier, R. M., & Freeman, J. B. (2015). Advanced mouse-tracking
analytic techniques for enhancing psychological science. *Group
Processes & Intergroup Relations, 18*(3), 384-401.

bimodality_coefficient for more information about the bimodality coefficient.

dip.test for more information about Hartigan's dip test.

```
# Calculate measures
mt_example <- mt_measures(mt_example)
# Assess bimodality for untransformed variables
mt_check_bimodality(mt_example,
use_variables=c("MAD", "AD"))
#> $BC
#> MAD AD
#> 1 0.6239962 0.7714127
#>
#> $HDS_p_value
#> MAD AD
#> 1 0.949304 0.9805709
#>
# Standardize variables per participant
mt_example <- mt_standardize(mt_example,
use_variables=c("MAD", "AD"), within="subject_nr")
# Assess bimodality for standardized variables
mt_check_bimodality(mt_example,
use_variables=c("z_MAD", "z_AD"))
#> $BC
#> z_MAD z_AD
#> 1 0.5099593 0.7212444
#>
#> $HDS_p_value
#> z_MAD z_AD
#> 1 0.7749003 0.7463675
#>
# Assess bimodality with simulated p values for HDS
mt_check_bimodality(mt_example,
use_variables=c("z_MAD", "z_AD"),
methods=c("BC", "HDS_sim"))
#> $BC
#> z_MAD z_AD
#> 1 0.5099593 0.7212444
#>
#> $HDS_simulated_p_value
#> z_MAD z_AD
#> 1 0.7855 0.753
#>
# Assess bimodality per condition
mt_check_bimodality(mt_example,
use_variables=c("z_MAD", "z_AD"),
grouping_variables="Condition")
#> $BC
#> Condition z_MAD z_AD
#> 1 Atypical 0.4056693 0.3430277
#> 2 Typical 0.4842767 0.7781194
#>
#> $HDS_p_value
#> Condition z_MAD z_AD
#> 1 Atypical 0.3458943 0.4716631
#> 2 Typical 0.9831558 0.9923261
#>
```