Summary
In this lesson students will learn about forensic impression evidence. They will come to understand the methods for documenting and analyzing two- and three-dimensional impressions by collecting dental stone castings and inked impressions of shoe prints. After analyzing the characteristics of these unknown samples, students will attempt to match them to known shoes. The lesson concludes with a discussion of the strengths and challenges of using impression evidence to make positive identification of suspects.
Essential Question(s)
How is forensic impression evidence collected and analyzed?
Snapshot
Engage
Students are introduced to the concept of impression evidence and how to collect and document three-dimensional impressions.
Explore
Students make casts of three-dimensional shoe-print impressions with dental stone.
Explain
Students learn about characteristics of impression evidence and analyze their three-dimensional impressions.
Extend
Students create two-dimensional impressions of their own shoes, and as a class attempt to match the impressions to corresponding shoes.
Evaluate
Students answer lab questions and as a class discuss the strengths and challenges of collecting and analyzing impression evidence.
Materials
Lesson Slides
Dental stone (2 lbs per group)
Digital scale
Plastic zip-top bags, gallon (1 per group)
Tongue depressors or spoons (1 per group)
Rulers
Black printer’s ink
Ink roller
White butcher paper
Shoes (1 per group for 3D; 1 per student for 2D)
Impressions Investigation handout (attached; one per student)
Lab Data and Questions handout (attached; one per student)
Preparation for the Lesson
Before beginning, edit slides 19 and 32 in the Lesson Slides with your classroom-specific instructions, as noted in the Explore and Extend sections below.
Engage
20 Minute(s)
Display slides 2–4 to introduce the essential question and learning objectives. Continue to slide 5 and present the crime scene scenario that sets up the first activity.
Display slides 6–10. These slides define impression evidence, provide a brief overview of categories of impression evidence, and focus specifically on three-dimensional impressions. After covering the content, tell students that they will be focusing specifically on shoe prints in this lesson, but the techniques for documenting other kinds of impression evidence are similar.
Continue to slides 11–12 to review with students how to document 3D impression evidence. Emphasize to students the importance of including a measuring device for scale to ensure that the evidence can be measured accurately. Point out that measurements should be triangulated from two fixed points. Next, go to slides 13–16 to show examples of photographs of the same impression taken from different flash angles.
To finish this section of the lesson, go to slide 17 and share with students how 3D impressions can be collected via dental stone. Tell students that they will be collecting their own dental stone impressions next.
Explore
60 Minute(s)
Group students into groups of 2 or 3 and provide each student with the Impressions Investigation handout. Have students use the “Dental Stone Casting - 3D Impression” section of the handout as you review the investigation procedures for casting the impression. Display slide 18 to show an abbreviated version of the casting instructions.
Go to slide 19 and give the class instructions for how they will proceed with the activity. Complete the dental stone castings and set them aside for now.
Explain
45 Minute(s)
Tell students they will be analyzing their 3D impressions and comparing them to the known shoe to determine if any of the impressions match. (Note that only one group’s impression will match the known shoe.)
Next, go to slide 20. Students will do a modified I Think, We Think strategy. For the “I Think,” have students in small groups generate a list of shoe features an investigator might look at in order to determine whether an impression matches a known shoe. Then come together as a class to share out those ideas as the “We Think.”
Go to slides 21–26 and review considerations, class characteristics, individual characteristics, and wear. Slide 25 has a zoomed-in comparison of an impression and matching shoe, with slide notes describing the individual characteristics numbered on the image. Next, distribute the Lab Data and Questions handout and display slide 27 to go over the data table format. In their investigation groups, have students document their 3D castings with a sketch and measurements, and record individual and class characteristics. While students work on this, pass the known shoe around the room and have students record the same information for it as well.
After completing their side-by-side comparison, have students answer the question beneath the Known Shoe - Outsole table:
Were you able to make a positive identification of the suspect’s shoe from your dental stone impression?
How did the data help you reach your conclusion?
Have the group with the matching impression explain their reasoning to the class.
Extend
45 Minute(s)
Students now practice making, collecting, and analyzing two-dimensional (2D) impressions and matching prints to their peers’ shoes.
Go to slides 28–30 and introduce two-dimensional impressions and documentation of evidence. Slide 30 has example photos of oblique lighting for reference.
Return to the lab instructions and review with students the investigation procedures under the 2D Inked Impression heading. Slide 31 has an abbreviated version of these instructions that you can display as well. Have each student make their impression, and label each impression with a unique letter.
After students have completed their impressions, redistribute the prints or have them take a known shoe. Go to slide 32 and give students instructions for how they should proceed with the activity. Have them document measurements and characteristics on their data sheet (or inked impression) as they did for the 3D impressions. Using the data, have students circulate the room to match the 2D impressions and shoes.
After completing their comparisons, have students answer the questions beneath the Inked Impression table:
Were you able to make a positive identification between a print and a shoe?
How did the data help you reach your conclusion?
Evaluate
30 Minute(s)
Bring the class back together after they have successfully matched their prints and shoes and ask them to answer the lab questions at the end of the data sheet. This can be done individually or collaboratively. Next, go to slide 33 and open up a discussion with the students about how they used evidence to draw their conclusions about whether an impression matched a shoe (i.e., what characteristics they used). Use slides 33–36 as a jumping off point to discuss the lab questions together. The goal is for students to come away understanding some of the strengths and challenges associated with collecting and analyzing impression evidence, and using it for identifying suspects.
Conclude the lesson by going to slide 37 and having students complete an “I Used to Think… but Now I Know…” reflection about forensic impression evidence.
Resources
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