Category Archives: Forensic controls

First, I refer you to Martin Yant’s earlier post on this subject:  https://wrongfulconvictionsblog.org/2012/05/10/cell-phone-evidence-doesnt-always-ring-true/

The post makes the point that data from a single cell tower is essentially worthless in trying to place someone in a particular location.  The best you can expect is a band within a 120° “pie wedge” from the cell tower.

If two cell towers are used, it gets much better, and if three towers are used it gets even better yet.  But to make sure this kind of evidence doesn’t get misused, and to know what it’s limitations are, it’s important to know how it works.

You may have noticed that the antennas on a cell tower are always arranged in a triangle.  There are some sound technical and economic reasons for this, but we won’t go into that here.  But it does mean that a cell tower can tell from which of the three antenna arrays it is receiving a signal.  Each of the three antenna arrays covers a 120° sector with the tower at it’s focus, and these sectors, by convention, are referred to as alpha, beta, and gamma – α, β, γ.

Within each sector, the tower can make a measurement of how far away the transmitting cell phone is.  This is done by measuring signal strength and the round-trip signal time.  For a lot of technical reasons, this is not a very accurate measurement, and the determined distance will have a reasonably significant error band.

Here is a diagram of a single cell tower showing concentric bands of distance from the tower, and the three “sectors”.  The distance bands don’t stop at “6”, but this is just to give you the idea.  Note that at six miles out, the arc of a sector is 12.6 miles long.

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Microscopic comparison of hair evidence is a long established forensic discipline.  Technically, it’s called “trichology”, and the field dates to the mid-1800’s.  Like other visual pattern matching forensics, it had come to be accepted despite the fact that it has essentially no basis in science – just empirical, anecdotal inferences drawn from flawed inductive reasoning.  Microscopic comparison of hair evidence has been used as the sole evidentiary basis to not only convict people, but also to execute them.  The problem is – it’s not reliable.  There are details of two cases cited below in which hair evidence was just plain wrong.

Here’s how it works.  This is a diagram of the basic structure of hair:

A hair from the crime scene, or one found on the victim, will be microscopically compared with a number of hair samples from the suspect.  The examiner will record observations of characteristics such as the following:

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From the Huffington Post:

A Texas judge who reviewed the controversial 2004 execution of Cameron Todd Willingham planned to posthumously exonerate the father who was put to death for killing his three daughters in a house fire.

Scientific experts who debunked the arson evidence used against Willingham at his 1992 trial and a jailhouse witness who recanted his shaky testimony convinced District Court Judge Charlie Baird in 2010 that “Texas wrongfully convicted” him. But Baird’s order clearing Willingham’s name never became official, because a higher court halted the posthumous inquiry while it considered whether the judge had authority to examine the capital case.

While waiting for permission to finish the case from the Third Court of Appeals, Baird put together the document that “orders the exoneration of Cameron Todd Willingham for murdering his three daughters,” because of “overwhelming, credible and reliable evidence” presented during a one-day hearing in Austin in October 2010.

“You can’t do anything for Willingham except clear his name,” Baird told The Huffington Post. “When they tried Willingham, I’m convinced that everyone Continue reading →

Daniel Vanek, DNA expert in the Czech Republic

I am in the Czech Republic lecturing over the next few days about the Innocence Movement.  I am so heartened and inspired when I go to countries like this to find kindred souls who are on the ground educating, teaching, and fighting for justice for the innocent.  DNA specialist Daniel Vanek is that person in the Czech Republic, and I am honored to be able to to spend time with someone who is this passionate about the movement.  I will write more later when I’m back to the States, but wanted to share now a video that was on national television in the Czech Republic this weekend about Daniel and his fight for forensic controls and the wrongfully convicted.

From source:

The Supreme Judicial Court of Massachusetts has decided to uphold the convictions in a first-degree murder prosecution even after finding that a prosecution witness gave mathematically erroneous DNA testimony.

“[O]ur review of the record indicates that Leanna Farnam, the chemist from the DNA unit of the State police crime laboratory who testified on behalf of the Commonwealth, made several calculation errors in her conversions of fractions into percentages, and thereby understated the likelihood that a randomly selected individual with no connection to the item could be ‘included’ as a potential contributor to the DNA samples,” Justice Margot Botsford wrote for a unanimous court.

“We conclude, however, that there was no substantial likelihood of a miscarriage of justice,” Botsford continued. “The difference of one per cent between the figures given by Farnam and the true figures was unlikely to have substantially influenced the jury’s deliberations, because the percentage of the population who were excluded as possible contributors to the DNA remained very high — over ninety-eight per cent — meaning it was still very unlikely that anyone included as a contributor had no connection to the item.”