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.
How does blood evidence link a perpetrator, victim, and scene together?
Use measurements to determine the angle of incidence of a blood drop.
Use rulers properly.
Each student should have:
- Bloodstain Student Investigation Sheet
- Metric Ruler
- Calculator with Trigonometry Functions
- On the blood spatter card, use a straight edge and draw a straight line bisecting the blood spatter ellipse lengthwise.
- Draw a straight line bisecting the blood spatter ellipse widthwise.
- 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.)
- Measure the widthwise bisecting line and record the spatter’s width in millimeters.
- Repeat steps 1–4 for each example.
- Divide the width measurement by the length measurement and record the answer.
- 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
- 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.
- Calculate the sine of each of the angles of incidence for each bloodstain spatter. Add it to your data table.
- What can the angle of incidence tell us about the crime being investigated?