## Overview

The goal of simChef is to help you quickly cook up a fully-realized, high-quality, reproducible, and transparently documented simulation study using an intuitive and tidy grammar of simulation experiments:

experiment <- create_experiment() %>%
dgp = dgp1,
n = c(100, 1000, 10000)
) %>%
method = method1,
lambda = c(0.1, 0.5, 1.0)
)

results <- experiment %>%
run_experiment()

### Installation

simChef is under active development. To install the package directly from GitHub, please use:

devtools::install_github("Yu-Group/simChef")

### Concepts

In simChef, simulation studies are decomposed into five intuitive concepts: experiments, data-generating processes, methods, evaluations, and visualizations. A simulation can either be contained in a single experiment or divided into multiple self-contained experiments which are like small simulations studies in their own right. Every experiment is in turn composed of four parts, two of which are optional (but highly recommended): data-generating processes (DGPs), methods, evaluation (optional), and visualization (optional).

simChef takes an object-oriented approach to encapsulate these simulation concepts, using R6 classes to make them concrete. The five main objects are:

• Experiment: corresponds to the experiment concept. As you can probably guess, this class is the main powerhouse of the simulation, collecting related DGPs and methods, keeping track of what parameters to vary, checkpointing and saving results, and producing evaluations metrics, visualizations, and documentation so that the simulation’s findings can be understood and easily communicated. Moreover, it uses future to compute experimental replicates in parallel using whatever resources you choose.
• DGP: corresponds to the data-generating process concept. DGPs simply generate synthetic data in a reproducible and flexible manner, in the size and manner that you specify. For a library of preset but highly customizable DGPs, including support for data-driven DGPs to give added realism to your synthetic data, simChef has a sibling R package, dgpoix (currently in early development).
• Method: corresponds to the method concept, which can be either a baseline, a target of the simulation study, or any means by which to transform the raw synthetic data. Together with DGPs, methods make up the main computational course of the simChef meal.
• Evaluator: corresponds to the evaluation concept. When computation of experimental replicates has completed, evaluators receive the results and summarize them to produce meaningful statistics about the experiment, or simply transform the results (e.g., using summary statistics). This is an optional step, but without it the experiment’s results can be much more difficult to understand and communicate.
• Visualizer: corresponds to the visualization concept. These visualizations can be applied directly to the raw experimental replicates’ outputs, can instead work with the evaluation transformations/summaries, or both. Visualizers can output anything that can be rendered in an R Markdown document: static or interactive plots, tables, strings and captured output, markdown, generic HTML, etc.

### Simulation study documentation

When all of this is put together, the Experiment class can output an R Markdown document that is structured to provide a well-organized summary of a simulation study. Moreover, this document can contain multiple experiments, simply by using a common output path with each Experiment in the study. When the simulation is complete, you can use the create_rmd() helper to generate the documentation:

create_rmd(experiment)

This results in an HTML document like the one shown below:

For more examples, including an interactive version of the simulation study documentation, see vignette("simChef").

## Origins of simChef

In their 2020 paper “Veridical Data Science”, Yu and Kumbier propose the predictability, computability, and stability (PCS) framework, a workflow and documentation for “responsible, reliable, reproducible, and transparent results across the data science life cycle”. Under the umbrella of the PCS framework, we began the process of deriving a set of guidelines tailored specifically for simulation studies, inspired by both high-quality simulation studies from the literature and our own simulations to examine the statistical properties of methods within the PCS framework.

While creating our own simulations, we soon found that no existing R package could fully satisfy our developing requirements. What began as a toolbox for our own simulations became simChef. We believe these tools will be useful for anyone intending to create their own simulation studies in R.

### Thinking like a chef

The development of simChef has been guided by our love of… cooking? Perhaps surprisingly, we found that cooking serves as useful analogy for the process of creating a simulation study. Consider the following components of a high-quality meal:

• Nutritious and delicious ingredients – All good meals start with good ingredients, and the same is true of simulation experiments. If realistic simulation data (entirely synthetic or driven by real-world data) is not available, then there is no hope of producing high-quality simulations. Creating realistic synthetic data is the primary goal of our sibling package dgpoix, which was initially integrated into simChef.
• Skill and experience of the chef – Just as every chef’s cooking is informed by the handful of cuisines in which they specialize, simulation experiments are motivated by scientific questions from a particular domain. Just as a chef does not have to become an expert knifemaker before cooking their first meal, nor should the domain scientist have to waste time writing boilerplate code to for the computation and documentation of their simulations. simChef takes care of the details of running your experiments across the potentially large number of data and method perturbations you care about, freeing up time for you to focus on your scientific question.
• High-quality tools in the kitchen – Our package should be like an excellent chef’s knife or other kitchen essential. If a chef’s knife doesn’t cut straight or isn’t sharpened, then kitchen speed and safety suffers, as does the final presentation. simChef won’t cook a good simulation experiment for you, but it will get you there with less effort and higher-quality presentation while helping you follow best-practices like reproducibility with minimal effort on your part. No sharpening required!
• A high-quality meal is possible in almost any environment – While the scale of a delicious meal may be limited by environment, high-quality meals are not only found in the world’s Michelin-starred restaurants but also in home kitchens and street food carts around the world. An effective simulation framework should also be agnostic to environment, and simChef runs equally well on your laptop as on a high-performance computing cluster.
• Appetizing and approachable presentation – Ultimately, a chef prepares food for a specific audience, and presentation is almost equal in importance to the underlying substance of the meal. However, a chef doesn’t have to build the plate on which they serve their food. simChef provides tools to turn your simulation experiment results into effective displays of quantitative information which are populated within preset and customizable R Markdown templates.

• Implement an abstract API to allow for a grammar of simulation experiments.
• Run experimental replicates in parallel and agnostic to computational backend via the R package future.
• Output an automated R Markdown report summarizing the results of an Experiment.
• Allow for varying simulation experiments across arbitrary parameters of DGPs and Methods.
• Cache results to avoid re-running already computed components of an Experiment.
• Checkpoint simulations to avoid losing progress in the case of unexpected problems, e.g. node failure.
• Gracefully handle errors from user-defined functions and return partial results error information for user to inspect upon completion.
• Incorporate progressr for simulation progress updates.
• Include a set of off-the-shelf DGPs (moved to dgpoix), Evaluators, and Visualizers to allow users to quickly run their methods in a number of common types of simulations.
• Allow for user customization of the final R Markdown report (e.g. a customized R Markdown template/theme, order of Evaluator and Visualizer displays).
• Give user the ability to choose which tasks are distributed to parallel workers, i.e. simulation replicates, DGPs, Methods, or combinations of the three.
• Enable nested parallelization, e.g. one may paralellize across DGPs using multiple nodes on a cluster and parallelize across simulation replicates using the CPU cores within each node.
• Publish to CRAN.

Below, we examine the main functionality of a number of existing tools for running reproducible simulation experiments that are currently available on CRAN and have been updated within the last couple of years.

• batchtools implements abstractions for “problems” (similar to our DGP concept), “algorithms” (Method in simChef), and “experiments”. In addition to shared-memory computation via the parallel and snow packages, it also provides a number of utilities for working with high performance computing batch systems such as Slurm and Torque, which simChef supports via the future.batchtools package.
• SimDesign provides helper functions to define experimental conditions and then pass those experimental conditions to a user-defined data generation function, analysis function, and summary function. The package also provides a number of these functions for the user to choose from. Each experimental condition can be run over many replicates, computing results in parallel via the parallel package.
• simhelpers defines functions to calculate Monte Carlo standard errors of simulation performance metrics, generate skeleton simulation code, and evaluate in parallel across simulation parameters via the future package.
• The simTool package has two main functions: expand_tibble() and eval_tibble(). The former wraps the base R function expand.grid() to create a cartesian product of simulation functions and parameters, while the latter evaluates those functions in parallel via the parallel package.
• The parSim package implements a single function of the same name which allows for parallelization of arbitrary R expressions across replicates and simulation conditions. parSim uses the snow package to setup parallel backends.
• rsimsum is an R implementation of the Stata command simsum and provides helper functions for summarizing and visualizing the results of a simulation study.