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Bloodstain Analysis: Student Investigation Sheet

Blood is among the basic types of physical evidence often encountered at a crime scene. Investigators use blood evidence to identify individuals by both blood type and DNA. By observing and analyzing patterns formed by blood at a scene, they can restage the location, movement, and actions of the crime. Like other physical evidence, blood evidence often links a scene, perpetrator, and victim and can be used to determine a sequence of events.


Essential question

How does blood evidence link a perpetrator, victim, and scene together?


Investigation objectives

Use measurements to determine the angle of incidence of a blood drop.


Safety

Use rulers properly.


Materials

Each student should have:

  • Bloodstain Student Investigation Sheet
  • Metric Ruler
  • Calculator with Trigonometry Functions


Procedures

  1. On the blood spatter card, use a straight edge and draw a straight line bisecting the blood spatter ellipse lengthwise.
  2. Draw a straight line bisecting the blood spatter ellipse widthwise.
  3. Measure the lengthwise bisecting line of the blood spatter ellipse and record the spatter's length in millimeters. (When measuring a spatter drop that has a thin tail, do not include the tail in the length measurement.)
  4. Measure the widthwise bisecting line and record the spatter's width in millimeters.
  5. Repeat steps 1–4 for each example.
  6. Divide the width measurement by the length measurement and record the answer.
  7. Calculate the angle of incidence for each drop by using the inverse sine function of a calculator or by looking up the value on a sine function chart. The inverse sine of width/length is the angle of incidence.


Blood spatter card


Data


Analysis

  1. Assume all of the blood spatters were found on a flat, horizontal surface. Draw an arrow through the blood spatter indicating the direction of blood flow.
  2. Calculate the sine of each of the angles of incidence for each bloodstain spatter. Add it to your data table.
  3. What can the angle of incidence tell us about the crime being investigated?


Analysis

Sample answers

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