Wednesday, May 31, 2017

A Few Statistical and Legal Ideas About the Weight of Evidence

The expression “weight of evidence” has become popular among theorists of forensic science, where it is used to indicate the extent to which findings support the claim that two similar traces originated from the same source as opposed to the claim that they originated from different sources. Speaking more broadly, the idea is that the degree of corroboration a body of evidence provides for a theory or hypothesis depends on the probability of the evidence given that hypothesis compared to the probability of the evidence given other hypotheses. This notion has a rich intellectual history in philosophy, law, and statistics.A recent book review* discusses ways to quantify this measure of corroboration and the motivations for them. Some excerpts follow:

The “likelihood ratio” is a concept that pervades statistics. 31/ As [Richard] Lempert argued, it can be used to define whether an item of evidence is [logically] relevant. For example, in the 1990s researchers developed a prostate cancer test based on the level of prostate-specific antigen (“PSA”). The test, they said, was far from definitive but still had diagnostic value. Should anyone have believed them? A straightforward method for validation is to run the test on subjects known to have the disease and on other subjects known to be disease-free. The PSA test was shown to give a positive result (to indicate that the cancer was present) about 70% of the time when the cancer was, in fact, present, and about 10% of the time when the cancer was not actually present. Thus, the test has diagnostic value. The doctor and patient can understand that positive results arise more often among patients with the disease than among those without it.

But why should we say that the greater probability of the evidence (a positive test result) among cancer patients than among cancer-free patients makes the test diagnostic of prostate cancer? There are three answers. One is that if we use it to sort patients into the two categories, we will (in the long run) do a better job than if we use some totally bogus procedure (such as flipping a coin). This is a “frequentist” interpretation of diagnostic value.

A second justification takes the notion of “support” for a hypothesis as fundamental. 37/ Results that are more probable under a hypothesis H1 about the true state of affairs are stronger evidence for H1 than for any alternative (H2) under which they are less probable. If the evidence were to occur with equal probability under both states, however, the evidence would lend equal support to both possibilities. In this example, such evidence would provide no basis for distinguishing between cancer-free and cancer-afflicted patients. It would have no diagnostic value, 38/ and the test should be kept off the market. The coin-flipping test is like this. A head is no more or less probable when the cancer is present than when it is absent.

A difference between the “frequentist,” long-run justification and the “likelihoodist,” support-based understanding is that the latter applies even when we do not perform or imagine a long series of tests. If it really is more probable to observe the data under one state of affairs than another, it would seem perverse to conclude that the data somehow support the latter over the former. The data are “more consistent” with the state of affairs that makes their appearance on a single occasion more probable (even without the possibility of replication).

The same thing is true of circumstantial evidence in law. Circumstantial evidence E that is just as probable when one party’s account is true as it is when that account is false has no value as proof that the account is true or false. It supports both states of nature equally and is logically irrelevant. To condense these observations into a formula, we can write:
E is irrelevant (to choosing between H1 and H2) if P(E|H1) = P(E|H2),
where P(E|H1) and P(E|H2) are the probabilities of the evidence conditional on (“given the truth of,” or just “given”) the hypotheses. The conditional probabilities (or quantities that are directly proportional to them) have a special name: likelihoods. So a mathematically equivalent statement is that
E is irrelevant if the likelihood ratio L = P(E|H1) / P(E|H2) = 1.
A fancier way to express it is that E is irrelevant if the logarithm of L is 0. Such evidence E has zero “weight” when placed on a metaphorical balance scale that aggregates the weight of the evidence in favor of one hypothesis or the other. 39/ In this [prostate cancer] case, the likelihood ratio for a positive test result is 70% ÷ 10% = 7, which is greater than 1. Thus, the test is relevant evidence in deciding whether the patient has cancer. ...

Nothing that I have said so far involves Bayes’s rule. “Likelihood” and “support” are the primitive concepts. Lempert argued for a likelihood ratio of 1 as the defining characteristic of relevance by relying on a third justification—the Bayesian model of learning. How does this work? Think of probability as a pile of poker chips. Being 100% certain that a particular hypothesis about the world is correct means that all of the chips sit on top of that hypothesis. Twenty-five percent certainty means that 25% of the chips sit on the same hypothesis, and the remaining 75% are allocated to the other hypotheses. 42/ To keep things as simple as possible, let’s assume there are only two hypotheses that could be true. To be concrete, let’s say that H1 asserts that the individual has cancer and that H2 asserts that he does not. Assume that doctors know that men with this patient’s symptoms have a 25% probability of having prostate cancer. We start with 25% of the chips on hypothesis 1 (H1: cancer) and 75% on the alternative (H2: some other cause of the symptoms). Learning that the PSA test is positive for cancer requires us to take some of the chips from H2 and put them on H1. Bayes’s rule dictates just how many chips we transfer. The exact amount generally depends on two things: the percentage of chips that were on H1 (the prior probability) and the likelihood ratio L in this simple situation. ... [T]he very simple structure of Bayes’s rule in this case [is]
Odds(H1) · L = Odds(H1|E).
The rule requires updating the “prior odds” (on the left-hand side) by multiplying by the Bayes factor (which also is the likelihood ratio L) to arrive at the “posterior odds” (on the right-hand side). ...

The crucial point is that multiplication by L = 1 never changes the prior odds. Evidence that is equally probable under each hypothesis produces no change in the allocation of the chips—no matter what the initial distribution. Prior odds of 1:3 become posterior odds of 1:3. Prior odds of 10,000:1 become posterior odds of 10,000:1. The evidence is never worth considering. Again, we can get fancy and place the odds and the likelihood ratio on a logarithmic scale. Then the posterior log odds are the prior log odds plus the weight of the evidence (WOE = log-L):
New LO = Prior LO + WOE. 44/
Evidence that has zero weight (L = 1, log-L = 0) leaves us where we started. Evidence E that does not change the odds (and, hence, the corresponding probability) is uninformative—it is irrelevant. Inversely, evidence that does change the probability is relevant—as [Federal] Rule [of Evidence] 401 states in near-identical terms. This, in a nutshell, is the Bayesian explanation of the rule as it applies to circumstantial evidence. It tracks the text of the rule better than the likelihoodist, support-based analysis, but both lead to the conclusion that relevance vel non turns on whether the likelihood ratio departs from 1. ...

... The simple likelihood ratio is the basic measure that dominates the forensic science literature on evaluative conclusions. However, most writers in this area construe the likelihood ratio as the ratio of posterior odds to prior odds and base its use on that purely Bayesian interpretation. Greater clarity would come from using the related term “Bayes factor” when this is the motivation for the ratio. 51/ [Note 51: The choice of words is not merely a labeling issue. In simple situations, the Bayes factor and the likelihood ratio are numerically equivalent, but more generally, there are conceptual and operational differences. For instance, simple likelihood ratios can be used to gauge relative support within any pair of hypotheses, even when the pair is not exhaustive. But when there are many hypotheses, the Bayes factor is not so simple. See [Peter M. Lee, Bayesian Statistics 140 (4th ed. 2012)], at 141–42. It becomes the usual numerator divided by a weighted sum of the likelihoods for each hypothesis. The weights are the probabilities (conditional on the falsity of the hypothesis in the numerator). For an example, see Tacha Hicks et al., A Framework for Interpreting Evidence, in Forensic DNA Evidence Interpretation 37, 63 (John S. Buckleton et al. eds., 2d ed. 2016). Furthermore, there is disagreement over the use of a likelihood ratio for highly multidimensional data (such as fingerprint patterns and bullet striations) and whether and how to express uncertainty with respect to the likelihood ratio itself. Compare Franco Taroni et al., Dismissal of the Illusion of Uncertainty in the Assessment of a Likelihood Ratio, 15 Law, Probability & Risk 1, 2 (2016), with M.J. Sjerps et al., Uncertainty and LR: To Integrate or Not to Integrate, That’s the Question, 15 Law, Probability & Risk 23, 23–26 (2016). ... ] ...

The obvious Bayesian measure of probative value is the Bayes factor (B). In the examples used here, B is equal to the likelihood ratio L, and therefore the statisticians’ “weight of evidence” is WOE = log-B = log-L. 58/ The value of L in these cases tells us just how much more the evidence supports one theory than another and hence—this is the Bayesian part—just how much we should adjust our belief (expressed as odds) for any starting point. For the PSA test for cancer, L = 7 is “the change in odds favoring disease.” A test with greater diagnostic value would have a larger likelihood ratio and induce a stronger shift toward that conclusion. ... [T]he likelihood-ratio measure (or variations on it), which keeps prior probabilities out of the picture, is more typically used to describe the value of test results as evidence of disease or other conditions in medicine and psychology. Using the same measure in law has significant advantages. ...

* David H. Kaye, Digging into the Foundations of Evidence Law, 115 Mich. L. Rev. 915 (2017)  (reviewing The Michael J. Saks & Barbara A. Spellman, Psychological Foundations of Evidence Law (2016)).

31. Vic Barnett, Comparative Statistical Inference 306 (3d ed. 1999) (“The principles of maximum likelihood and of likelihood ratio tests occupy a central place in statistical methodology.”); see, e.g., id. at 178–80 (describing likelihood ratio tests in frequentist hypothesis testing); N. Reid, Likelihood, in Statistics in the 21st Century 419 (Adrian E. Raftery et al. eds., 2002).

37. A “support function” can be required to have several appealing, formal properties, such as transitivity and additivity. E.g., A.W.F. Edwards, Likelihood 28–32 (Johns Hopkins Univ. Press, expanded ed. 1992) (1972). It also can be derived, in simple cases, from other, arguably more fundamental, principles. E.g., Barnett, supra note 31, at 310–11.

39. See generally I. J. Good, Weight of Evidence and the Bayesian Likelihood Ratio, in the Use of Statistics in Forensic Science 85 (C.G.G. Aitken & D.A. Stoney eds., 1991); I. J. Good, Weight of Evidence: A Brief Survey, in 2 Bayesian Statistics 249 (J.M. Bernardo et al. eds., 1985) (providing background information regarding the use of Bayesian statistics in evaluating weight of evidence). ...

42. If the individual were to keep some of the chips in reserve, the analogy between the fraction of them on a hypothesis and the kind of probability that pertains to random events such as games of chance would break down.

44. A deeper motivation for using logarithms may lie in information theory, but, if so, it is not important here. See Solomon Kullback, Information Theory and Statistics (1959).

58. The logarithm of B has been called “weight of evidence” since 1878. I.J. Good, A. M. Turing’s Statistical Work in World War II, 66 Biometrika 393, 393 (1979) ... . While working in the town of Banbury to decipher German codes, Alan Turing famously (in cryptanalysis and statistics, at least) coined the term “ban” to designate a power of 10 for this metaphorical weight. Good, supra, at 394. Thus, a B of 10 is 1 ban, 100 is 2 ban, and so on.

Saturday, May 20, 2017

Science Friday and Contrived Statistics for Hair Comparisons

On May 19th, Public Radio International's Science Friday show had a segment entitled "There’s Less Science In Forensic Science Than You Think." The general theme — that some practices have not been validated by rigorous scientific testing — is a fair (and disturbing) indictment. But listeners may have come away with the impression that the FBI has determined that hair examiners make up statistics from personal experience 95% of the time to help out prosecutors.

Ira Flato, the show's host, opened with the observation that "The FBI even admitted in 2015, after decades, investigators had overstated the accuracy of hair sample matches over 95% of the time in ways that benefited the prosecution." He returned to this statistic when he asked Betty Layne DesPortes, a lawyer and the current President of the American Academy of Forensic Sciences, the following question:
Dr. DesPortes, I want to go back to that FBI admission in 2015 that for decades investigators had overstated the accuracy of their hair samples, and I mean 95% of the time in a way that benefited the prosecution. Is this a form of cognitive bias coming into the picture?
Ms. DesPortes replied that
It is, and ... you would have overstatement along the lines of, "Well, I’ve never seen in my X years of experience that two hairs would be this similar, so it must be a match," and then they would just start making statistics up based on, "Well, I’ve had a hundred cases in my practice, and there have been a thousand cases in my lab, and nobody else has ever reported similar hairs like this," so let’s just start throwing in one in a hundred thousand as a statistic — "one in a hundred thousand" — and that’s where the misstatement came in.
But neither Ms. DesPortes nor anyone else knows how often FBI examiners cited statistics like "one in a hundred thousand" based on either their recollections of their own casework or their impression of the collective experience of all hair examiners. 1/

To be sure, such testimony would have been flagged as erroneous in the FBI-DOJ Microscopy Hair Comparison Review. But so would a much more scientifically defensible statement such as
The hair removed from the towel exhibited the same microscopic characteristics as the known hair sample, and I concluded it was consistent with having originated from him. However, hair comparison is not like fingerprints, for example. It’s not a positive identification. I can’t make that statement." 2/
The Hair Comparison Review was not designed to produce a meaningful estimate of an error rate for hair comparisons. It produced no statistics on the different categories of problematic testimony. The data and the results have not been recorded (at least, not publicly) so as to allow independent researchers to ascertain the extent to which FBI examiners overstated their findings in various ways. See David H. Kaye, Ultracrepidarianism in Forensic Science: The Hair Evidence Debacle, 72 Wash. & Lee L. Rev. Online 227 (2015).

The interim results from the Hair Comparison Review prompted the Department of Justice to plan a retrospective study of FBI testimony involving other identification methods as well. In July 2016, it asked a group of statisticians how best to conduct the new "Forensic Science Disciplines Review." The informal recommendations that emerged in this "Statisticians' Roundtable" included creating a database of testimony that would permit more rigorous, social science research. But this may never happen. A new President appointed a new Attorney General, who promptly suspended the expanded study.

  1. Ms. DesPortes may not have meant to imply that all the instances of exaggerated testimony were of the type she identified.
  2. That statements like these may be scientifically defensible does not render them admissible or optimal.
(For related postings, click on the label "hair.")

Tuesday, May 16, 2017

The Reappearing Rapid DNA Act

With bipartisan sponsorship, the Rapid DNA Act of 2017 (H.R.510 and S. 139) is sailing through Congress. The Senate bill made it to the legislative calendar on May 11, 2017, without amendment and without a written report from the Judiciary Committee.  The Committee Chairman, Senator Grassley, wrote this about the bill:
Turning to legislation, the first bill is S.139, the Rapid DNA Act of 2017. It is sponsored by Senator Hatch. The Committee reported this bill and the Senate passed it in the last Congress. The bill would establish standards for a new category of DNA samples that can be taken more quickly and then uploaded to our national DNA index. 1/
This characterization is misleading. The bill itself contains no standards for producing profiles to upload to the national database. It orders the FBI to “issue standards.” Specifically, the part of the bill entitled “standards” adds to the DNA Identification Act of 1994, 42 U.S.C. § 14131(a), a new Section 5, which reads as follows:
(A) ... the Director of the Federal Bureau of Investigation shall issue standards and procedures for the use of Rapid DNA instruments and resulting DNA analyses.
(B) In this Act, the term ‘Rapid DNA instruments’ means instrumentation that carries out a fully automated process to derive a DNA analysis from a DNA sample. 2/
But the FBI does not need new authorization to devise standards for “Rapid DNA instruments.” The “resulting DNA analyses” are not a new category of “samples,” and some such profiles already may be in the National DNA Index System (NDIS). In fact, the FBI issued standards for “rapid” profiles years ago. One need only peek at the FBI's forthright answers to “Frequently Asked Questions on Rapid DNA Analysis.” There, the FBI explained that
Based upon recommendations from the Scientific Working Group on DNA Analysis Methods (SWGDAM), the FBI Director approved and issued The Addendum to the Quality Assurance Standards for DNA Databasing Laboratories performing Rapid DNA Analysis and Modified Rapid DNA Analysis Using a Rapid DNA Instrument (or “Rapid QAS Addendum”). The Addendum contains the quality assurance standards specific to the use of a Rapid DNA instrument by an accredited laboratory; it took effect December 1, 2014.
The FBI added that “[a]n accredited laboratory participating in NDIS may use CODIS to upload authorized known reference DNA profiles developed with a Rapid DNA instrument performing Modified Rapid DNA Analysis to NDIS if [certain] requirements are satisfied” and that “DNA records generated by an NDIS-approved Rapid DNA system performing Rapid DNA analysis in an NDIS participating laboratory are eligible for NDIS.” 3/

But if the FBI does not need the bill to develop standards or to incorporate rapid-DNA results into NDIS, what is the real purpose of the bill? The answer is simple. The bill clears the way for these results to come, not from accredited laboratories, 4/ but from police stations, jails, or prisons. The House Judiciary Committee was explicit in its brief report on the bill:
Currently, booking stations have to send their DNA samples off to state labs and wait weeks for the results. This has created a backlog that impacts all criminal investigations using forensics, not just forensics used for identification purposes. H.R. 510 would modify the current law regarding DNA testing and access to CODIS. The short turnaround time resulting from increased use of Rapid DNA technology would help to quickly eliminate potential suspects, capture those who have committed a previous crime and left DNA evidence, as well as free up current DNA profilers to do advanced forensic DNA analysis, such as crime scene analysis and rape-kits. 5/
The FBI was more succinct when it referred to “the goal of using Rapid DNA systems in the booking environment” and reported that “legislation will be needed in order for DNA records that are generated by Rapid DNA systems outside an accredited laboratory to be uploaded to NDIS.6/

Is the migration of DNA profiling from the laboratory to the police station — and potentially to the officer on the street — a good idea? The efficiency argument from the House Committee has some force. We do not demand that only accredited laboratories conduct breath alcohol testing of drivers who seem to be intoxicated. Police using properly maintained portable instruments can do the job. 7/

How is DNA different? In one respect, it is less problematic than roadside alcohol testing. Rapid DNA analysis is not for crime-scene samples. (At least, not yet.) It is for samples from arrestees or convicted offenders whose profiles can be uploaded to a database. The police have an incentive to avoid uploading inaccurate profiles. Such profiles will degrade the effectiveness of the database. Any cold hits that they might produce will be shown to be false when a later DNA test from the suspect fails to replicate the incorrect profile. In contrast, incriminating output of a faulty alcohol test usually enables a conviction and will not be shown to be in error.

But there is more to the matter than efficiently generating and uploading profiles. It could be argued that DNA information is more private that a breath alcohol measurement and that having CODIS profiles known to local police is more dangerous than having it known only to laboratory personnel. Considering the limited kind of information that is present in a CODIS profile, however, this argument does not strike me as compelling.


The Rapid DNA Act of 2017 met no opposition as the Senate and House passed the bills. S. 139 generated unanimous consent (and no discussion) on May 16. 8/ Its counterpart, H.R. 510, passed after receiving praise from two of its sponsors and the observation from Representative Goodlatte (R-VA) that "this is a good bill. It is a bipartisan bill. I thank Members on both sides of the aisle for their contributions to this effort." 9/

  1. Prepared Statement by Senator Chuck Grassley of Iowa, Chairman, Senate Judiciary Committee Executive Business Meeting, May 11, 2017,, viewed May 16, 2017.
  2. Rapid DNA Act of 2017, S. 139 § 2(a).
  3. The difference between “Rapid DNA Analysis” and “Modified Rapid DNA Analysis” is that the former is “a “swab in – profile out” process ... of automated extraction, amplification, separation, detection, and allele calling without human intervention,” whereas the latter uses “human interpretation and technical review” for ascertaining the alleles in a profile. FBI, Frequently Asked Questions on Rapid DNA Analysis,, Nos. 1 &2, viewed May 17, 2017.
  4. The DNA Identification Act of 1994, 42 U.S.C. § 14131, which the Rapid DNA Act amends, requires the FBI to create and consider the recommendations of "an advisory board on DNA quality assurance methods." § 14131(a)(1)(A).  The members of the board must come from "nominations proposed by the head of the National Academy of Sciences and professional societies of crime laboratory officials." Id. They "shall develop, and if appropriate, periodically revise, recommended standards for quality assurance, including standards for testing the proficiency of forensic laboratories, and forensic analysts, in conducting analyses of DNA." § 14131(a)(1)(C). As the name indicates, the board is purely advisory. The Act only demands that
    The Director of the Federal Bureau of Investigation, after taking into consideration such recommended standards, shall issue (and revise from time to time) standards for quality assurance, including standards for testing the proficiency of forensic laboratories, and forensic analysts, in conducting analyses of DNA.
    § 14131(a)(2).
    The advisory board was a half-a-loaf response to the recommendation of a National  Academy of Sciences committee for "a National Committee on Forensic DNA Typing (NCFDT) under the auspices of an appropriate government agency, such as NIH or NIST, to provide expert advice primarily on scientific and technical issues concerning forensic DNA typing." NRC Committee on DNA Technology in Forensic Science, DNA Technology in Forensic Science 72-73 (1992). Now that NIST has established an Organization of Scientific Area Committees for Forensic Science to develop science-based standards for DNA testing and other forensic science methods, Congress should reconsider the need for the overlapping FBI board.
  5. On May 11, 2017, the House Committee on the Judiciary recommended adoption of H.R. 510 without holding hearings. The Judiciary Committee saw no need to consult independent scientists. It was satisfied with the fact that
    the Judiciary Committee’s Subcommittee on Crime, Terrorism, Homeland Security and Investigations held a hearing on a virtually identical bill, H.R. 320, on June 18, 2015, [at which] testimony was received from: Ms. Amy Hess, Executive Assistant Director of Science and Technology, Federal Bureau of Investigation; Ms. Jody Wolf, Assistant Crime Laboratory Administrator, Phoenix Police Department Crime Laboratory, President, American Society of Criminal Laboratory Directors; and Ms. Natasha Alexenko, Founder, Natasha’s Justice Project.
    Report to accompany H.R. 510, May 11, 2017,
  6. FBI Answers, No. 13,, viewed May 17, 2017 (emphasis added).
  7. “As of January 1, 2017, there is no Rapid DNA system that is approved for use by an accredited forensic laboratory for performing Rapid DNA Analysis.” Several systems had been approved but they do “not contain the 20 CODIS Core Loci required as of January 1, 2017.” FBI Answers, No. 6,, viewed May 16, 2017. 
  8. 163 Cong. Rec. S2954-2955, 115th Cong., 1st Sess., May 16, 2017.
  9. Id. at H4205.

Saturday, May 6, 2017

Who Copy Edits ASTM Standards?

This posting is not about science or law. It is about English writing. I recently had occasion to read the “Standard Guide for Analysis of Clandestine Drug Laboratory Evidence” issued by ASTM International, a private standards development organization. The standard exemplifies a common problem with the ASTM standards for forensic science — an apparent absence of copy and line editing to achieve clear and efficient expression of the ideas of the committees that write the standards. 1/

This particular standard, known as E2882-12, opens with an observation about the “scope” of the document — namely, that
This guide does not replace knowledge, skill, ability, experience, education, or training and should be used in conjunction with professional judgment.
The word “replace” has caused a couple of readers to complain that this admonition implies that unstructured “knowledge, skill, ability, experience, education, or training” suffices for the analysis of the evidence. That is not  a fair reading of the sentence, but joining the two clauses with “and” makes it seem like they are separate points. Why not make it as easy as possible for the reader to get the intended message? I think the sentence amounts to nothing more than the following simple idea:
This standard is intended to help professionals use their knowledge and skill to analyze clandestine drug laboratory evidence.
Why not just say this? Why all the extra verbiage?

Unfortunately, this text is not an isolated example of the need for detailed editing. Another infelicity is
capacity—the amount of finished product that could be produced, either in one batch or over a defined period of time, and given a set list of variables.
The words “and given a set list of variables” are a sentence fragment. They dangle aimlessly after the comma. The copy edit is obvious:
Capacity is the amount of finished product that could be produced, for a specified set of variables, either in one batch or over a stated period of time.
It still may not be clear what a “set of variables” means here, but at least the words about unnamed variables occur where they belong.

The wording in a section on reporting is especially obscure:
Laboratories should have documented policies establishing protocols for reviewing verbal information and conclusions should be subject to technical review whenever possible. It is acknowledged that responding to queries in court or investigative needs may present an exception.
One clear statement of what the sentences seem to assert is that
Laboratories should have written protocols to ensure that oral communications from laboratory personnel are reviewed for technical correctness. However, a protocol can dispense with (1) review of some courtroom testimony and (2) review that would impede an investigation.
Whether this edited version expresses what the authors wanted to say or presents a satisfactory policy is unclear, but at least the version is more easily understood.

Other phrases that should raise red flags for editing abound. I’ll end with three examples.
  • This guide does not purport to address all of the safety concerns, if any, associated with its use. The editor would say: Make up your mind. If there are no safety concerns, then the sentence is worthless. If there are safety concerns, then the standard should address them. If there is a reason not to address all of them, then the standard can say, “There are additional safety concerns for a laboratory to consider.” If there is a desire to be very cautious, it could read, “There could be additional safety concerns for a laboratory to consider.”
  • ... calculations can be achieved from ... . Copy editor: It sounds odd to speak of "achieving" calculations. The phrase "calculations can be made by" would be more apt.
  • Quantitative measurements of clandestine laboratory samples have an accuracy which is dependent on sampling and, if a liquid, on volume calculations. This sentence is both circumlocutious ("which is dependent") and disjointed ("if a liquid" is in the wrong place to modify "samples"). It also seems to conflate measurements on subsamples of the material submitted for analysis ("clandestine laboratory samples") with inference from the subsamples to the sample of the seized items. If this reading of the dense sentence is correct, editing would expand it along the following lines: "The accuracy of quantitative measurements of a liquid sample depends on the calculated volume of the sample. When the material analyzed is not the entire sample, then the accuracy of any inferences to the entire sample also depends on the homogeneity of the sample and the procedure by which the subsample was chosen.
Good writing requires the right words in the correct order. Good editing makes the writing more readable. Many existing technical standards in forensic science still need good editing to make them fully fit for purpose.

  1. Although some publishers distinguish between line editing and copy editing, this posting uses the phrase "copy editing" broadly, to refer to the process of reviewing and correcting written material to ensure "that whatever appears in public is accurate, easy to follow, and fit for purpose." Society for Editors and Proofreaders, FAQs: What Is Copy-editing?,