Positive Predictive Value (PPV)
Definition
The probability that a person who tests positive on a risk instrument will actually reoffend. PPV depends on both the instrument's sensitivity and specificity and the base rate of reoffending in the population assessed. A key target of sophisticated Daubert cross-examination of actuarial risk testimony.
- Definition context
- In forensic assessment, PPV is the probability that a positive result on a risk test reflects a true positive case
- Key influence
- PPV is heavily dependent on base rate, sensitivity, and specificity of the test instrument
- Legal relevance
- A key target of Daubert and Frye challenges to actuarial risk testimony in expert witness cross examination
Common questions
What does it mean when a risk test has high PPV?+
High PPV means that when someone scores positive on the test, there is a strong likelihood they will actually exhibit the behavior being predicted. PPV depends on both how well the test works (sensitivity and specificity) and how common the behavior is in the population being tested.
Why does base rate matter for PPV?+
The base rate is how often a behavior actually occurs in the population. Even if a test is very good at identifying positives and negatives, PPV changes based on base rate. In a population where reoffending is rare, a positive test is less predictive than in a population where it is common. This is why courts scrutinize whether a risk instrument's PPV holds true for the specific population in the case.
How is PPV different from sensitivity?+
Sensitivity tells you how many actual cases the test catches (what fraction of true positives does it find). PPV tells you, when the test says positive, how often it is right. Both matter for assessing a test's value, but they answer different questions. Sensitivity depends only on the test itself, while PPV also depends on base rate and specificity.
Related terms
- BSA 2023 § 39
- Section 39 of the Bharatiya Sakshya Adhiniyam 2023, the provision governing expert opinion evidence in Indian courts. Replaced Indian Evidence Act 1872...
- Daubert Standard
- The US federal evidentiary standard (Daubert v. Merrell Dow Pharmaceuticals, 1993) requiring that expert testimony be based on scientifically valid methods with...
- AUC Statistic
- Area under the receiver operating characteristic curve; the standard criterion-validity metric for forensic risk and malingering instruments, with 0.70 considered a minimum...
- Base Rate
- The prior probability of an event or proposition before specific evidence is considered. In forensic inference, the base rate is the probability...
- BSA 2023 § 63
- The electronic record admissibility provision (replacing IEA § 65B), requiring a certificate from a person in a responsible official position confirming the...
- Consequential Validity
- The fairness and equity implications of test use; includes disparate impact on racial or cultural groups and the systematic bias this may...
- CrimPR Part 19
- Criminal Procedure Rules Part 19 (England and Wales): the expert-witness procedural code requiring disclosure of qualifications, methodology, limitations, and whether the method...
- Criterion Validity
- The degree to which test scores predict or correlate with an external criterion; predictive criterion validity is most relevant to forensic instruments.
- Ecological Validity
- The degree to which a measurement or test procedure produces results that generalise from the controlled laboratory context to the real-world condition...
- Frye Standard
- The US legal test for admissibility of scientific evidence, originating from Frye v. United States (1923), which required that a technique be...
- Incremental Validity
- The improvement in predictive accuracy that a test adds over and above prior information; justifies the cost and burden of additional assessment.
- Reliability
- Consistency of measurement: test-retest, inter-rater, and internal-consistency coefficients all quantify different aspects of this.
Explained in these topics
- Forensic-Psychology Expert Witness and Daubert / Frye ChallengesThe probability that a person who tests positive on a risk instrument will actually reoffend. PPV depends on both the instrument's sensitivity and specificity...
- Foundations of Forensic Assessment and Test ValidityThe probability that a positive test result reflects a true positive; heavily dependent on base rate, not just on sensitivity and specificity.