Computer simulation can be a very powerful tool in a variety of fields, from accident reconstruction and biomechanics to fire investigation and product design. But computer simulations can also be full of unrealistic assumptions and use invalid techniques. If an opposing expert relies on a simulation, how do you know if they are using reliable, valid, scientific techniques or junk science that should be excluded before trial?
Animation or Computer Simulation?
Determining whether the opposing expert has created an animation or a computer simulation may not be obvious initially, but the distinction is important for how the court will evaluate its acceptability. An animation is usually a video demonstrating an expert’s opinion or a witness’ description of an incident. Therefore, an animation is just a demonstrative exhibit. In contrast, a computer simulation involves calculations that ultimately form part of the basis for the expert’s opinion. The computer simulation may also produce a video showing the results of the simulation, so it may require some investigation to determine if a video comes from a computer simulation or just an animation.
An expert produces a video showing two vehicles approaching an intersection. The video is used to demonstrate how far from the intersection the vehicles were as they approached impact. This is an animation because it is only being used for demonstrative purposes.
Figure 1. Example animation of vehicles at an intersection.
An expert produces a video of two cars crashing in an intersection. The cars impact each other and move to their respective points of rest. In this case, the physics of how the cars impact each other, how much damage results, and how they move after the impact is based on the results of the computer program. Therefore, this is a computer simulation.
Figure 2. LS-Dyna finite element model simulation of a vehicle to vehicle collision.
Acceptability of Simulations in Court
Determining if an animation is acceptable in court or not is relatively straightforward. The animation must be an accurate representation of the facts of the case without being overly prejudicial to the jury. A computer simulation is more complicated. Computer simulations should be broken into two parts: the calculations and results of the simulation are the first part and the resulting video (if one exists) is the second part. Once the computer simulation is broken into these two parts, the analysis can be evaluated like any other analysis using Rule 702, Daubert, or the applicable law in the jurisdiction.
Rule 702 states in part that an expert may testify if:
- the testimony is the product of reliable principles and methods
- the expert has reliably applied the principles and methods to the facts of the case
Daubert clarifies that the following factors can help determine whether the testimony is scientific:
- whether the theory or technique employed by the expert is generally accepted in the scientific community
- whether it has been subjected to peer review and publication
- whether it can be and has been tested
- whether the known or potential rate of error is acceptable
- whether the research was conducted independent of the particular litigation or dependent on an intention to provide the proposed testimony
Accepted Scientific Methods – Simulation Validation
Evaluation of an expert’s validation is a key step in determining whether their simulation follows generally accepted scientific methods. Validation is defined as “the process of determining the degree to which a model is an accurate representation of the real world from the perspective of the intended uses of the model.” Therefore, “regardless of the use, confidence in computational simulations is only possible if the investigator has verified the mathematical foundation of the model and validated the results against sound experimental data.” 
In a New York supreme court case, a computer simulation was excluded because the technique was not generally accepted by the scientific community. Specifically, in the product liability action, the plaintiff’s expert had created an alternative design and demonstrated it using a computer simulation. The computer simulation had not been validated by any real-world testing. The defense expert testified that using a computer simulation without validation is not an accepted scientific technique.
The “intended uses” part of the definition is also particularly important. Often an expert will use a commercially available computer model and say that “it has been validated.” However, has the model been validated for the intended use? For example, I worked on a case where the opposing expert used a Madymo computer simulation to investigate the likelihood of a thoracic spine fracture. Madymo has a suite of models that have been validated to represent many aspects of the human body, and are used to simulate human movement and injury risk for many body regions. However, the risk of thoracic spine fracture was not one of them. Therefore, the model was not validated for the expert’s intended use and was therefore not using a method accepted by the scientific community.
Figure 3. Images of Madymo human models (left) and dummy models (right).
Reconstruction vs. Illustrating a General Principle
There are multiple ways that computer simulations can be used to support an expert’s opinion. Most uses can be categorized into reconstructions and simulations illustrating a general principle. Either category can be evaluated in relation to Part (d) of Rule 702 which indicates that an expert must apply their method to accurately represent the facts of the case. In the case of a reconstruction, an expert must appropriately support all of their simulation modeling choices as representative of the subject case. This can be very difficult to accomplish as almost all simulations must make assumptions or simplifications relative to reality. For example, consider a simulation of an automobile accident used to demonstrate the effectiveness of a seat belt in preventing an occupant’s injuries. A reconstruction would need accurate vehicle dynamics of the subject accident; accurate body weight and dimensions of the occupant; and accurate properties for the seat belt, steering wheel, instrument panel, etc. Such a reconstruction is likely very difficult to match all the facts of the case with accuracy and precision, and therefore could be subject to attack for any aspects of the simulation that do not match the subject case. On the other hand, a simulation illustrating the general principle of seat belt effectiveness in a similar collision may not be subject to the same expectations. However, if the simulations are not close enough to the subject case, they may still be excluded. How the simulation is presented and what question(s) it is meant to answer for the jury is an important piece of evaluating the acceptability of the simulation.
Figure 4. LS-Dyna simulation of a frontal impact with a seatbelt and airbag.
Process for Evaluating a Computer Simulation
Once you have determined that an opposing expert has used a computer simulation as opposed to animation, the first step is to identify what type of software was used, or if custom software was used. There are numerous types of computer simulations an expert can perform from finite element analysis to computational fluid dynamics and multi-body physics to modeling chemical and biological processes (Table 1). Each type of computer simulation has many commercial and custom software packages associated with them. Once you identify the type of software and any commercial version information, find an expert in that method to evaluate the simulation.
Common Computer Simulation Software Types and Commercial Packages
Accident Reconstruction and Biomechanics
- HVE (Human Vehicle Environment)
- ARAS 360
- ATB (Articulated Total Body)
- GEOBOD (Generator of Body Data)
Finite Element Analysis (FEA)
Computational Fluid Dynamics (CFD)
- Ansys Fluent
Fire, Smoke, and Evacuation Simulation
- FDS (Fire Dynamics Simulator)
- PHAST (Process Hazard Analysis Software)
An expert in the specific type of software is necessary for several reasons. First, most computer simulation files are specific to the software used and may not be possible to open without specialized software. Second, experts do not always readily produce all files associated with simulations. For example, in a recent case, we requested disclosure of all input and output files associated with the computer simulations performed. The expert disclosed only 3 of the 35 files produced by the simulation software. In a deposition, the expert claimed to have deleted all the other files. Without an expert, the attorneys would not have known the disclosure was incomplete or how to address the insufficient disclosure. Be prepared to file motions to get complete disclosure. Remember that any computer simulation is just a complicated set of calculations. You would not let an expert testify to results of calculations without disclosing what equations he or she used and what numbers went into the equations. Disclosing all input and output files for the computer simulation is the only way to get that information.
If an opposing expert has relied on a computer simulation, it is important to evaluate the simulation to determine if the expert used reliable methods that appropriately represent the facts of the case. If not, the simulation may be excluded. However, evaluating the simulation is not a trivial endeavor and will require an expert with specialized knowledge of the simulation software used as well as careful disclosure requests to get all information related to the simulation.
 Webster, Victoria, and Fred E. Bourn III. “The Use of Computer-Generated Animations and Simulations at Trial.” Def. Counsel J. 83 (2016): 439.
 Federal Rules of Evidence, Rule 702, December 1, 2014.
 Daubert v. Merrell Dow Pharmaceuticals, Inc., 43 F. 3d 1311 – Court of Appeals, 9th Circuit 1995
 Henninger, Heath B., et al. “Validation of computational models in biomechanics.” Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine 224.7 (2010): 801-812.
 Lascano v. Lee Trucking, 2007 N.Y. Misc. LEXIS 6872.
 MADYMO Human Body Models Manual, Release 7.5, June 2013. P29-30.
 Aldous, Alan. “Disclosure of expert computer simulations.” Computer/LJ 8 (1987): 51.