The evidence analysis circuit that closes the investigation into the death of Marcus Bell, 34.
By the end of this block you will say whether the death of Marcus Bell was an accident or foul play, and you will back that with at least four exhibits drawn from at least three different evidence types.
Time: 80 minutes, in class. Every station is timed. You will not finish if you read ahead.
Forensic laboratories in the United States handle hundreds of thousands of evidence submissions a year, and the single most common reason a result is thrown out in court is not bad chemistry. It is a missing control. A test nobody can check is a test nobody has to believe.
Today is five short investigations, not five worksheets. Each one has the same three parts, and the whole point of the block is that you can name those parts by the end of it. This is the part of today that will be on next week's exam.
In one sentence of your own, what is a control group for?
A classmate says, "my result looks clean, so I do not need the control." Write the one sentence you would say back.
No two people have ever been found to share a complete fingerprint, which is why prints have been used to identify people for over a century. In the same century, wrongful convictions have also turned on print examiners who were sure and wrong. Both facts are true, and they are why the procedure below makes you classify before you compare.
A print lifted from the scene is on the left of EXHIBIT FP-1. Five reference prints are on file. You work in two passes: classify the pattern, which narrows the crowd, then compare minutiae, which is the only step that can point at one finger.
Read this before you decide anything: a print proves that a finger touched a surface. It does not say when, and it does not say why. Some of the people on file have an ordinary reason to have touched the front edge of the mixing console.
Your control is the victim's own reference print, printed on the reverse of your evidence sheet. Marcus Bell lived and worked at this scene, so that print should appear on scene surfaces. Check that your procedure finds it before you trust your procedure on anybody else. A method that cannot find the print you know is there has not earned an opinion about the prints you are unsure of.
| Reference print | Ridge patternarch, loop or whorl | Same pattern as the scene print?yes / no | Marks in agreementout of 3 |
|---|---|---|---|
| T. Cole | |||
| R. Vasquez | |||
| D. Okoye | |||
| L. Tran |
Which reference print could the scene print have come from, and which step told you: naming the ridge pattern, or comparing the marks?
That person has a reason to have touched the front edge of the mixing console that has nothing to do with this death. Write it. Then write what the print does and does not prove.
A presumptive blood test can find blood that has been wiped, washed and painted over, at dilutions far below what an eye can see. That sensitivity is why investigators still swab a scene that looks clean, and the number you measure today is exactly the number that decides whether a faint pink result means anything at all.
This is the only wet lab today. You are making a serial dilution of hydrochloric acid and using phenolphthalein, the same indicator molecule that sits inside the presumptive blood test used on this case, to find how far you can dilute something and still detect it.
A serial dilution is the same dilution done over and over. Each tube takes 1.0 mL from the tube before it into 9.0 mL of water, so each tube is one tenth the concentration of the one before it. Five tubes gets you from the stock down to one hundred thousandth of it, using nothing but a pipette.
Two control tubes are already made up and already in your rack.
Check both before tube 1 and again at the end. Record both checks in the table.
| Tube | Dilution from stockfactor | Concentrationmol/L | Drops of base to reach pinkdrops | Colour before basepink / colourless |
|---|---|---|---|---|
| C1 | ('control',) | ('0.1 M base',) | ('none needed',) | |
| C2 | ('control',) | ('water only',) | ('none needed',) | |
| 1 | ('1:10',) | ('0.010',) | ||
| 2 | ('1:100',) | ('0.0010',) | ||
| 3 | ('1:1,000',) | ('0.00010',) | ||
| 4 | ('1:10,000',) | ('0.000010',) | ||
| 5 | ('1:100,000',) | ('0.0000010',) |
Each step is ten times weaker than the one before. What happened to the drop count at each step, and does the pattern match that?
Which tube was the last one where you could still tell something was there? That is your detection limit. Write the tube and its concentration.
Exhibits E2-console and E2-floor gave a faint pink on the presumptive test, and the two controls on file, E2-ctrl-soda and E2-ctrl-rust, are there because both can also turn a presumptive test pink. Using your detection limit, write one sentence on what a faint pink does prove and one on what it does not.
DNA profiling has freed more than two hundred wrongly convicted people in the United States and has identified thousands of others. It does both jobs with the same property: fragments of different lengths travel different distances through a gel. Everything below is that one idea, used carefully.
DNA is cut into fragments, loaded into wells at the top of a gel, and pulled through by an electric field. DNA carries a negative charge, so it travels toward the positive end. Short fragments travel further than long ones, because the gel is a mesh and small pieces slip through it faster.
Lane 1 is a ladder: fragments of known size, printed on the figure. You use the ladder to build a standard curve, and then read every unknown band off that curve. That is the whole method.
Two control lanes are already on your gel and they do two different jobs. Look at both before you read a single sample lane.
| Lane | Distance to band 1mm | Distance to band 2mm | Size of band 1bp | Size of band 2bp |
|---|---|---|---|---|
| Victim reference | ||||
| Scene swab S-1, console edge | ||||
| Scene swab S-2, cabinet handle | ||||
| T. Cole | ||||
| R. Vasquez | ||||
| D. Okoye | ||||
| No-template control |
Do the two scene swabs share every band with the victim reference? Write which bands match and which do not.
One scene lane carries an extra band that the victim does not have. Which references also carry that band? Name every one of them.
You have one shared band and more than one person who carries it. Write the one sentence that says honestly what this gel can and cannot establish about who else touched that item.
Bloodstain pattern analysis answers a question no other evidence at a scene can: where in the room was the person when it happened, and how high off the floor. That single number, the height of the origin, is often the difference between a fall and a blow.
A drop that falls straight down lands round. A drop that arrives at a shallow angle lands stretched into a long ellipse, and it stretches in a predictable way. Measure the short axis and the long axis and the ratio gives you the angle it arrived at.
The four stains in EXHIBIT BS-1 were recorded on the front face of the mixing console and are drawn at the scale they were measured at, so your ruler works directly on the page.
Three control stains were dropped on purpose, from one fixed height, onto the same surface, at angles we set to 90, 60 and 30 degrees. Only the angle changed, so the angle is the independent variable and the shape of the stain is the dependent variable. Their measurements are printed for you. Check the formula against these three before you measure a single stain from the scene.
| Stain | Widthmm | Lengthmm | Width / lengthto 2 dp | Angle of impactdegrees |
|---|---|---|---|---|
| Control 90 | ||||
| Control 60 | ||||
| Control 30 | ||||
| A | ||||
| B | ||||
| C | ||||
| D |
Which stain arrived closest to straight down, and which arrived at the shallowest angle? Give the stain letter and its angle for each.
Your case pack records that these stains sit between 1.3 and 1.5 metres above the floor. Was the person upright or already on the floor when these stains were made? Answer, then give the measurement that made you say so.
Write the one thing this mini lab cannot tell you, no matter how carefully you measured.
Emergency physicians and crash investigators use the same energy arithmetic you are about to use, because the human body responds to energy delivered, not to how frightening an object looks. An object that seems harmless can carry enough energy to break bone, and a heavy object that barely moved cannot.
The speaker cabinet at this scene ended up on the floor across part of the body. The question is whether it could have caused the fatal injury, and that is an arithmetic question before it is anything else.
An object held above the floor stores energy. When it falls, that stored energy is delivered on impact. Energy equals mass times gravity times height. Mass and height come from the case record. The multiplication is yours.
These two numbers are from the case record and are given to you. Do not estimate them and do not re-derive them.
The reference band of published injury thresholds is also given. You place your own answer on it.
| Quantity | Value | Unit |
|---|---|---|
| Mass of the speaker cabinet | ('38.0',) | ('kg',) |
| Gravity | ('9.8',) | ('m/s squared',) |
| Height of fall | ('1.10',) | ('m',) |
| Energy delivered (m x g x h) | ('joules',) | |
| Injury range this lands in | ('from the band',) |
Write your energy value and the injury range it landed in.
Look at your graph. When you doubled the height, what happened to the energy? Write the relationship in one sentence.
Can this object have caused a fatal injury? Answer yes, no, or cannot tell from this alone, and give the number that decided it.
A medical examiner has to put one word on a death certificate: natural, accident, suicide, homicide, or undetermined. Undetermined is a real and honest answer, and using it when the evidence has run out is a professional act, not a failure.
You now have five results. Put them together. The question this case has been asking since Day 1 is the one printed below.
Did the speaker cabinet cause the fatal injury as it fell, or was the injury already there when the cabinet came down?
The rule for your verdict paragraph: at least four exhibits, drawn from at least three different evidence types, and every claim stops where the evidence stops. If two exhibits disagree, say so. A disagreement you name is worth more than a conclusion you force.
The claim ceilings for this case, unchanged since Day 1. Every one of them limits something you produced today.
Fill the evidence line first. One row per exhibit you are going to use.
| Exhibit | Evidence type | What it establishes | What it cannot establish |
|---|---|---|---|
Your verdict, in one sentence. Accident, foul play, or undetermined.
Your reasoning. Name each exhibit as you use it. If two exhibits pull in different directions, say which and why you weighted one over the other.
The one test you would run next, and exactly what result would change your mind.
| width / length | angle of impact (degrees) |
|---|---|
| 0.17 | 10 |
| 0.26 | 15 |
| 0.34 | 20 |
| 0.42 | 25 |
| 0.50 | 30 |
| 0.57 | 35 |
| 0.64 | 40 |
| 0.71 | 45 |
| 0.77 | 50 |
| 0.82 | 55 |
| 0.87 | 60 |
| 0.91 | 65 |
| 0.94 | 70 |
| 0.97 | 75 |
| 0.98 | 80 |
| 1.00 | 90 |
Round your ratio to the nearest value in the table.
STATIC (JH-26-0831-A) is a John Hay original teaching case written for this classroom. The scene, the persons of interest, the statements and every measurement in this packet were authored for instruction. No part of it is a real person, a real record, or a real death. Nothing produced in this packet can be used to make a claim about anyone outside this case.