Behind the Expert Testimony in the ‘Fitbit Murder’ Trial

This week, a Connecticut jury convicted Richard Dabate of murder in the murder of his wife, Connie Dabate, after a five-week trial that relied – in part – on data from her Fitbit. Richard said a man dressed in camouflage broke into their home in 2015 and shot Connie. But Connie was wearing a Fitbit and data from the device showed movement for about an hour after Richard said the break-in had occurred.

To prove that the Fitbit data helped show that Richard killed Connie, prosecutors turned to Keith Diaz, an exercise physiologist and professor at Columbia University Medical Center. Diaz has conducted studies to validate the accuracy of Fitbits and testified about their precision.

Diaz often testifies as an expert witness in criminal cases. He told The edge that it is personally satisfying to participate in trials, but that it is also challenging. The way he usually thinks about Fitbit data as a scientist is different from the way he’s asked about it in a courtroom. “The scientific questions we answer are different from the criminal questions,” he says. “What I’ve tried to do in these cases is translate that.”

Fitbits are generally accurate devices, Diaz says, but they’re certainly not perfect. For scientific research, that’s expected – nothing is ever absolute, and it’s rare to get an answer that comes with 100 percent certainty. Science lives with a certain amount of statistical error. But the law operates on different guidelines: it wants to know beyond a reasonable doubt whether something has happened. “This has very serious consequences if someone could go out for life for 25 years,” he says. “So it’s okay to live in a gray area — and under what conditions? That was part of the challenge and the hurdles to overcome.”

Diaz has spoken to The edge about his experiences with witnesses in the court cases and how he circumvents the tensions between science and the legal system.

The following has been slightly modified for clarity.

What’s the difference between what a scientist would want to know about a Fitbit and what a jury should know?

In science, we care about things – like I took 100 steps, but the device says I only took 92 steps. That matters in science, but who cares in criminal cases? It is often a matter of whether the device detects movement. That’s good enough for the business. It doesn’t matter if it’s 100 steps or 92; it’s really about whether they moved. In this Connecticut case, there was a lot of back and forth in the cross-examination about the error rate. But the error rate was the number of steps someone took — not the error rate of whether they were moving. That’s a whole different story.

And in science, especially in many of the studies I work with, you work with large samples that can hold thousands of people. Any sound is dispelled with so many people. But with a criminal case, you only have one person – so noise is important and can cast doubt on what really happened. In criminal cases, there’s a degree of precision that you want to have, okay, who was moving at this exact moment. Compared to science, if there is an error because you have such a large sample size, it can be filtered out. So it was a challenge to figure out how to make it clear that these devices can’t tell you perfectly what’s happening.

How do you tailor science to the needs of a criminal case?

You can take advantage of the things science might consider mistakes. So if you brush your teeth and wear a device on your wrist, there are instances where the device has misregistered as steps. In science, that is clearly a mistake. But that can be really helpful in some of these cases as it shows that it is a high sensitivity device. On the contrary, it is a strong point to use these devices in this context because of their high sensitivity. When no motion is detected, when no steps are recorded, we have to be pretty sure that a person is not moving.

So we had to change what we looked at in science to say, “Well, what’s most important in the context of this criminal case?” We can say that this person stopped moving because the device was not recording anything. So what does that mean in this context?

There are two sides to a criminal trial, and you are asked to testify on one side or the other. What’s that like as a scientist?

I was hired by prosecutors, never by the defense. And some prosecutors have gone out of their way to influence my interpretation. It may be unintentional, but they will have the perspective that they think this person is guilty and that they need someone to come in and support the case. But I don’t want to just go in and be ammunition for the prosecution; I want to give a balanced interpretation of what science means and how it can be applied in the situation. It might be easy for me to go in and say things that would support the prosecution’s case. But how do you balance that with also acknowledging that there is a flaw and that it is not a perfect device?

It’s difficult because the prosecution clearly doesn’t want to highlight the mistakes. But from the defense I noticed that they don’t ask the right questions about that. They are not scientists, so they do not know what to ask to emphasize the limitations of the device. And I can only answer the questions that are asked of me.

Has this experience changed your view of your research?

If this is something I go into further, I’d like to do more scientific studies. In many of the situations I’ve seen with these cases, the existing science isn’t really designed to answer the important questions. Such as: is it possible for a person to get from a bed to a bathroom 15 meters away without the device registering? So I might look into that sort of thing because I think it would bolster my ability to provide expert testimony. And it would be helpful for a jury to have better answers to these kinds of questions that come up in a criminal case that don’t exist in science. But for now it’s kind of a side gig. This is a bit outside of my day job.

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