dRiftDM

The package dRiftDM was developed to assist psychology researchers in applying and fitting diffusion models to empirical data within the R environment. Its most important feature is the ability to handle non-stationary problems, specifically diffusion models with time-dependent parameters (such as a drift rate that changes over the course of a trial). The package includes essential tools for standard analyses, such as building models, estimating parameters for multiple participants, and creating/visualizing summary statistics. The pre-built models available in the package are:

• The Standard Ratcliff Diffusion Model (Ratcliff, 1978, Psychological Review)
• The Diffusion Model for Conflict Tasks (Ulrich et al., 2015, Cognitive Psychology)
• The Shrinking Spotlight Model (White et al., 2011, Cognitive Psychology)

We can flexibly create custom models and utilize the dRiftDM machinery to estimate them. Variability in the starting point and non-decision time is generally supported. Variability in the drift rate is also supported, although it is limited to models with a time-independent/constant drift rate. Model components for a collapsing boundary are already pre-built. Demos for model customization are provided in one of our vignettes.

Model predictions (i.e., first-passage times) are derived by numerically solving the Kolmogorov-Forward equation (KFE) using a time-adaptive procedure (i.e., the time-step size is adjusted dynamically and this leads to substantial speed-up). This is the default, and we currently recommend keeping it that way.

For backward compatibility, it is also possible to derive the first-passage-time distribution via a coupled set of integral equations. However, this method does not yet support adaptive time stepping. Thus, finer discretization settings are necessary, and it will generally run slower than the approach based on the KFE.

Mathematical details of both approaches are provided in Richter et al. (2023, Journal of Mathematical Psychology).

Installation

You can install the development version of dRiftDM from GitHub with:

# install.packages("devtools")
devtools::install_github("bucky2177/dRiftDM")

The CRAN version can be installed with:

install.packages("dRiftDM")

Notes

Compared to the initial version 0.1.1, versions >=0.2.0 make greater use of the S3 object system. Additionally, beginning with version 0.2.0, models use flex_prms objects to handle parameters across conditions.

In versions >=0.3.0, we implemented a more efficient KFE-based solver, which led to a substantial speed-up. We also added several new features:

• different optimization algorithms (including unbounded Nelder-Mead and BFGS)
• different cost functions (with the root-mean-square error, RMSE, as an alternative to the log-likelihood)
• default parameter ranges for pre-built models
• visualizing the full distribution of a model and calculating basic summary statistics
• calculating/visualizing uncertainty intervals via bootstrap-like procedures
• fitting a model to aggregated data using the RMSE statistic
• fitting a model via hierarchical and non-hierarchical Bayesian estimation (currently experimental and not fully connected to all of dRiftDM’s features)

To install one of the older versions (0.1.1, 0.2.1, 0.2.2), you can use:

devtools::install_github("bucky2177/dRiftDM", ref = "<insert version>")

How to use dRiftDM

If you are interested in getting started with dRiftDM, we recommend reading the OSF pre-print. More information on functions and model customization can be found in dRiftDM’s vignettes. These vignettes are also available from the “Getting started” and “Articles” tabs on our Github.io page.

If you have any questions, feel free to contact us!