Saturday, July 14, 2012

Going South with Shoeprint Testimony

On July 9, 10, and 12 ("If the Shoe Fits, You Must Not Calculate It"), I discussed the much maligned opinion in R. v. T., [2010] EWCA Crim. 2439. Professor Mike Redmayne kindly called to my attention another Court of Appeal opinion on shoe print evidence: R. v. South, [2011] EWCA Crim. 754. It indicates that an expert  who does not use the adjective "scientific" can present the "verbal equivalent" of a likelihood ratio when the precise value of the ratio is uncertain. Indeed, the witness can give a source probability as long as it is a personal judgment derived solely from individual experience rather than from systematically collected data on shoe prints. This situation is not the best of all possible worlds.

Students in Bournemouth found that their house had been burgled in the afternoon as two of them slept. “On the floor below the letterbox of the front door there were some envelopes which had footmarks on them. These envelopes were subsequently given to the police and they were forensically examined. The evidence concerning those footprints was adduced at the trial.” This was by no means the only evidence the police developed against Sergio South, who was known to them as a burglar, but it became fodder for his appeal.

The evidence from an FSS examiner (a Mr. Jones) resembled that of the FSS's Mr. Ryder in R. v. T. Both experts, quite reasonably, relied on size, pattern, and wear. Here, "this footprint was in agreement with the size, pattern, detailed alignment and degree of wear with the trainer of the appellant that had been seized from him upon arrest. The zigzag bar pattern and the curved tramline were similar, and the trainers, which were size 9, were consistent with the footprint which was of size 9 or 8 but not size 10."

As in R. v. T., the expert must have consulted the FSS likelihood ratio table of “verbal equivalents.” Defense counsel “submitted that ... Mr Jones had said that the evidence relating to the footprint was ‘moderately strong support’ for the proposition that the appellant's shoe had made the imprint on the envelopes.” The phrase “moderately strong” is reserved in the FSS table for likelihood ratios between 100 and 1000—the next rung up the ladder from the “moderate support” for the ratio of 100 in R. v. T.

So what distinguishes the cases? Surely not that South's feet were smaller or the likelihood ratio larger. Is it that Mr. Jones was asked on cross-examination where the verbal equivalent came from?  Defense counsel advised the Court of Appeal that "Mr Jones had said that this expression reflected a statistical probability of the footprint having been made by the shoes of the appellant which was considerably more than a 50 per cent probability, because the linguistic phrases used, such as 'weak or limited support' or 'extremely strong support', were based on probability which was itself based on a logarithmic scale."

If this description of the cross-examination is correct, the expert's testimony was less defensible that that in R. v. T.  It appears that Mr. Jones missed the point of the FSS's efforts to train analysts in estimating likelihood ratios. The raison d'etre for using likelihoods to arrive at a standardized expression for the strength of the evidence is to get away from testimony about source probabilities. Statements such as "a statistical probability of the footprint having been made by the shoes of the appellant ... was considerably more than a 50 per cent probability" are strictly verboten. The expert following the strength-of-evidence approach must confine himself to commenting on the degree to which the evidence supports the competing claims about the source of the impressions. It is the role of the jury, and not the business of the expert, to consider the probability of those claims. In addition the expert (or the defense counsel) did not appreciate the fundamental difference between probabilities (of hypotheses about the origin of the marks) and likelihood ratios (which measure the support the evidence gives to those hypotheses). Whether this foggy cross-examination satisfied R. v. T.'s call for more transparency about the origin of an expert's description of the strength of the trace evidence is questionable.

Nevertheless, and even though Mr. Jones testified as "a scientist" who "had worked as in this area since 1982," the court concluded that his presentation "did not transgress in any way the guidelines set down by this court in R v T." The crucial fact for the court was that "Mr Jones' evidence was based on his experience." The South court described R. v. T. as stating “that if a footwear examiner expressed a view that went beyond saying that the footwear could or could not make the mark concerned, the report should make it clear that the view is subjective and based on experience of the examiner, so that words such as ‘scientific’ used in making evaluations should not in fact be used because they would, before a jury, give an impression of a degree of precision and objectivity which is not present given the current state of expertise.”

That Mr. Jones referred to "a statistical probability" did not seem to worry the court. Neither did the court perceive any problem with testimony "that he encountered the type of footwear seized from the appellant in only 2 per cent of cases that he dealt with as a forensic examiner of footwear and footprints" and "that burglars frequently used sports trainers." If "2 per cent" is a summary of 27 years of unrecorded personal experiences, it is hardly a rigorously ascertained "statistical probability," although it is a statistic and it yields a probability. And if Mr. Jones's understanding of the sartorial preferences of burglars informed his perception of "moderately strong" trace evidence yielding a posterior probability of "considerably more than 50 per cent," then he was exceeding the bounds of his expertise as a careful observer of similarities and differences and a keen analyst of the significance of these similarities and difference in impressions.

In sum, South indicates that the strictures of R. v. T. are easily avoided. But the courts and the forensic science profession do better. They can implement a system of reporting and testifying that conveys opinions or information in terms of the strength of the evidence rather than the probability of source hypotheses. There is considerable support for this approach among forensic service providers in Europe. The English courts lag behind, as do both the forensic science profession and the courts in the United States.


  1. I must ask what the difference between these two statements is that makes you think Europe is on a better track than the US (although England's use of likelihood ratios makes me agree with that statement):
    "This pair of shoes could have made the impression found at the crime scene. Other shoes with the same wear, pattern, and size characteristics could also have made the impressions."
    "Due to the wear, pattern, and size characteristics, it is more likely that these shoes made the impression than a pair of random shoes pulled from the general public."

    The top is more probative than the bottom in my opinion, because the bottom statement doesn't really say much. More likely than a random person isn't very useful. The mere fact they've been charged should key the jury into the fact there was a higher chance than a random person. Only these shoes and other shoes like it is more useful. Nobody has numbers for either the number of random shoes or the number of shoes like those in question, so both will result in a question nobody can answer until massive databases can be collected.

    Secondly, any use of likelihood ratios gathered only from crime scenes and bookings is entirely biased and should not be used.

  2. The first statement does not say much. It seems to be little more than an assertion that the defendant's shoes and all other similar shoes could have produced the evidence. Don't you have to add something about how large the set of similar shoes is within a population of plausible suspects?

    The second statement is not a statement about likelihoods. It is a statement about source probabilities -- an assertion that, given the features of the shoes and the mark, the probability that the defendant's shoes made the mark exceeds the probability that a selected randomly pair of shoes did.

    I would prefer a third option directed at the weight of the evidence -- perhaps that defendant's shoes would produce marks like the ones observed (somewhat) (considerably) more often than would a pair of shoes drawn at random from the shoes of all plausible suspects.