Fingerprinting:   A Lesson on Classification

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Guiding Question: Can we invent a way to classify fingerprints?

Facts:

• The patterns of ridges on our finger pads are unique: no two individuals—even identical twins—have fingerprints that are exactly alike.
• We leave impressions—or prints—of these patterns on everything we touch with any pressure.
• The prints can be visible, as when our fingers are dirty or oily, or they can be latent, as when they are made only by the sweat that is always present on our finger ridges.
• Injuries such as burns or scrapes will not change the ridge structure: when new skin grows in, the same pattern will come back.
• Dactyloscopy is the practice of using fingerprints to identify someone.

Principles:

• Fingerprints can be classified by pattern types, by the size of those patterns, and by the position of the patterns on the finger.

Skills:

• Observing
• Comparing and contrasting
• Classifying

Materials:

• 3x5-inch index cards, at least two per participant
• pencils and a sharpener
• transparent tape; 3/4-inch is better than 1/2-inch
• good lighting
• hand magnifiers—nice to have but not essential

Today we are going to look at some of our fingerprints and see how we might sort them into categories, just as fingerprint specialists do.

• Divide into groups of 2
• Rub a pencil over the central part of an index card until it is covered with graphite.
• Use the second card for recording your prints.  Write your name on the lined side and turn it over.
• Your will be making prints of the index finger and the middle finger of the same hand. Begin by asking who is right- or left-handed and  use that hand.
• Note: You want to make prints not of their fingertips but of the pads of their fingers, near the joint crease, because that is where the most interesting patterns are.
•  To use the graphite pads, press and roll your finger firmly on the penciled area, then stick a short piece of tape to the finger pad area, pressing down thoroughly, remove the tape and press it onto your print record card.
• Immediately label your print "L" or "R" for left or right hand and "I" or "M" for index or middle finger.
• Repeat procedure for the second finger. Do it over until you get two good prints.
• After all prints are made and labeled, compare your prints for similarities and differences.  Record your results in your lab notebook
• Are the two prints from the same hand more alike than prints from different people? How?  Record
• What are the positions of those patterns on the finger (how close they are to the joint line)?  Record
• What kinds of patterns do they see?  Give names to the patterns (circles, triangles, curvy lines)  Record
• In which direction do the loops curve—toward the thumb or toward the pinkie finger? (Remember that taped prints are like looking at your finger palm-up and inked prints are mirror images. It may be easier to ask whether they curve toward the right or left of the card.)   Record
• Compare the size of those patterns (such as how many ridges make up a loop).   Record
• After you have compared and named your prints, look at the handout with examples of "official" names for patterns (loops, whorls, and arches).  Record the official name of your prints in your Lab note book.
• Get with another Lab group and classify your prints according to the handout.  Record
• Pass your prints to another group and see if they agree with your classification.
• Get the results from the other groups.  Which is the most common pattern?  Make a bar graph of your results in your lab notebook.

Evaluation:  Answer the following in your lab note book.

• How can fingerprints be classified?
• How would classification make it easier to match one print against a database of many?