What do expert witnesses need to know about the preparation and use of diagrams, models, and maps at trial?

Attorney Timothy S. Tomasik explains:


Diagrams, models and maps, have nearly the same foundation requirements as videos and photographs.  Generally, diagrams, models, and maps are admissible at the sole discretion of the trial court judge if they fairly and accurately represent what they are purported to depict.  United States v. Hardin, 710 f.2d 1231 (7th Cir. 1983); see also Lake County Forest Preserve Dist. v. Vernon Hills Development Corp., 85 Ill.App.3d 241 (1980).  Tangible and anatomical models, like a replica heart or skeletal model, are also admissible at the discretion of the trial court.  See Flanagan v. Redondo, 595 N.E.2d 1077 (Ill. App. 1992).  The foundation for an anatomical model of a knee or an arm is typically the same as a photograph or videotape in that it must only be a fair and accurate depiction of a knee or arm.  See Id.  Notably, when the model depicts the anatomy in question, such as the Plaintiff’s knee or arm, then a stricter standard of review is required.  See Id.  Opposing counsel is much more likely to object to a depiction of the actual anatomy in question rather than a stock model.

This type of demonstrative evidence can be used to highlight a witness’ location at the time of an accident, belief as to what his/her property line was, or to establish the basis for an expert witness’ opinion.  If the diagram is not to scale, the witness should be able to testify to a demonstrative if it fairly and accurately depicts the scene or image.  However, if the diagram is to scale, then the witness must be prepared to answer the following types of questions:

–        How did you prepare the diagram/model/map?

–        What is the scale?

–        Can you describe for the members of the jury what “to scale” means?

–        Would showing the members of the jury this diagram help in explaining your investigations and opinions in this case?

–        Is this model (helmet, wrench, bottle, etc…) is the same or substantially similar condition as the original?

An example of expert testimony using a to-scale diagram or model is scaffold or platform design.  If admitted as an expert witness under either the weaker Frye (Illinois) standard or the heightened Daubert (Federal) standards, these witnesses should be prepared to go into much more detail regarding the creation of their map/model if it is part of the basis of their opinion.  See Fed. R. Evid. 702; IL. R. Evid. 702; See also Frye v. United States, 293 F. 1013 (D.C. Cir. 1923); Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993).  An engineering expert who created a to-scale diagram of a train platform must be able to testify as to his/her background in creating platform models, the scale of this particular model, and that it accurately depicts the construction of a platform.  It is essential to remember that the trial court judge has discretion when admitting diagrams or models like described above as irrelevant, prejudicial, or simply not reliable.  See Fed. R. Evid. 401, 403, 901.


Attorney Karen Koehler (The Velvet Hammer) will be speaking on Demonstrative Evidence and Expert Witnesses at the SEAK National Expert Witness Conference on May 3-4, 2014 in Orlando, FL.