MODEL-1: a finished comparison on a practice print

Part 3 of 4 · Worked example

Read it for the order of the work

This example uses a locker print that belongs to neither case. Its counts are supplied; nobody recounted an image. Read it for the order of the work, because Part B asks you to repeat that order with your own case's counts: features first, then the 12-match rule, then the percentage, then the limit.

The supplied inputs

Pattern family

Right loop

Readable minutiae

16

Card 1

13 matching, 0 mismatching

Card 2

5 matching, 3 mismatching

The 12-match rule, from R5: a model-individual call needs at least 12 matching minutiae and zero mismatching minutiae. Any mismatch makes the comparison inconsistent. Fewer than 12 matches with no mismatch is inconclusive. Every print call today uses this one rule.

The five moves, in order

1. List the features before considering the card's identity: right loop and 16 readable ridge details. A pattern family is a class feature.
2. Compare card 1's supplied count with the 12-match rule: 13 is at least 12, and the record reports zero mismatches. Under this exercise model, this supports the model's “individual” call, consistent with card 1. It is not automatic proof of identity or an action.
3. Compare card 2's supplied count with the same model. The record includes three mismatches. Under the 12-match rule, the comparison is inconsistent with card 2. This is an exercise conclusion, not a real forensic exclusion based only on counts.
4. Show the card 1 proportion: 13 ÷ 16 × 100 = 81.25%, or about 81% to the nearest whole percent. This percentage describes matching features in the record, not the probability that card 1's source touched the locker.
5. Keep the limit beside the comparison. A match in this classroom comparison supports a possible source under the 12-match rule; it does not by itself prove who touched the object, when, or why.

What the finished entry looks like

MODEL-1 reports a right loop with 16 readable minutiae. Card 1 has 13 matches and zero mismatches, meeting the 12-match rule; 13 of 16 is about 81%. Card 2 has five matches and three mismatches, giving an inconsistent comparison under that same model. These practice counts support bounded comparisons, not proof of who touched the locker or when.

A counterfactual, for practice only

The original model also asks you to imagine nine of 16 matching details with zero mismatches. That gives 56.25%, about 56%, and is below the classroom model's 12-match requirement. This counterfactual is practice only. Do not copy its numbers into B3, which supplies its own values.

Do not copy the model's numbers. B3 supplies its own: 14 readable, 9 matching, 0 mismatching. The model's 81% belongs to the model.

Connection to PLTW

Related PLTW reference:

• Activity 1.1.3 The Traces We Leave Behind

In Lesson 1.1, Investigating the Scene. This John Hay classroom adaptation practices describing evidence, comparing available information and explaining limits.

This sheet does not replace or certify completion of the official activity. If assigned, open the matching activity through your existing myPLTW account or section hub.