11/24/16

1

LING 411Topics in Applied Phonetics

SUPPOSE: a particular disease has an incidence of 1 in 10,000 people

a test for this disease is 99% accurate

(i.e., it correctly detects the disease 99% of the time and gets it wrong 1% of the time)

YOU:test positive for the disease

Q: What is the approximate probability that you actually have the disease?

a) 99%b)90%c) 10%d) 1%

11/24/16

2

SUPPOSE: a particular disease has an incidence of 1 in 10,000 people

a test for this disease is 99% accurate

IF WE TEST A GROUP OF 1 MILLION PEOPLE

100 will actually have the disease (1/10,000) 99 will be correctly diagnosed (99%)

AND

999,900 will not have the disease, but 9999 false positives (1%) will occur

total_true_positivestotal_positives

= 999999 + 99

= .0098 ≈1%

The chance you actually have the disease is a bit less than 1%.

11/24/16

3

Vocal Stress Analysis

• a speech-based “lie detector”

• based on the claim that certain acoustic patterns can be seen when the speaker is not telling the truth

Polygraph (“Lie Detector”)

è supposedly detects lying through measures of heart rate, galvanic skin response, blood pressure, breathing

• no solid scientific evidence has confirmed its accuracy

• people can be taught “countermeasures” to generate false negatives

• people can be taught to generate a false positive

• actual risk of wrongful convictions is unknown

• police often justify its use on the grounds of bogus pipeline effects

11/24/16

4

Polygraph (“Lie Detector”)

• results are not admissible as evidence in Canadian courts

• however, they ARE used by investigators as a forensic tool

• and by other agencies as a screening tool

Why?

Lie Detection through Voice Analysis?

Commercial Products

CVSA: Computerized Voice Stress Analyzer

LVA: Layered Voice Analysis

11/24/16

5

Vancouver SunMarch 2011 645 x $250 > $160,000

Vancouver SunMarch 2011

11/24/16

6

‘The LVA software claims to be based on 8,000 mathematical algorithms applied to 129 voice frequencies’ (Damphousse et al. 2007: 15)

... the LVA is a very simple program written in Visual Basic. The entire program code, published in the patent documents (Liberman 2003) comprises no more than 500 lines of code. ... With respect to its alleged mathematical sophistication, there is really nothing in the program that requires any mathematical insights beyond very basic secondary school mathematics.

From Ericksson & Lacerda, 2007

Damphousse et al (2007)

Assessing the Validity of Voice Stress Analysis Tools in a Jail Setting

Validity: Does the test do what is claimed?

Reliability: Does the test give the same result when it is administered more than once (e.g., by different testers)?

11/24/16

7

Damphousse et al.

random sample of new male arrestees (< 48 hrs)

Interviews

15 - 20 minutesCVSA requires yes/no questions

LVA requires ‘conversational style’

Ss were told that the study was examining the value VSA in assessing stress, though deception was not specifically mentioned (they probably guessed this)

Interviews

arrestees were asked about recent drug use (various drugs, up to 30 days for marijuana)

Ss knew the questions in advance (to reduce failed memory and telescoping effects, which proved to be

a problem anyway)

11/24/16

8

Interviews

telescoping: tendency to incorrectly recall time of a past event as more (or less) recent than it really

was

Post-interview

• Blind analysis of VSA results collected from

• Expert VSA analysts (many years of interrogation experience)

• Novice analysts (the ‘research team,’ who had no interrogation experience)

• Ss were asked to submit a urine sample for analysis

11/24/16

9

In reality, X is a liar

Test indicates Result is

Liar hit (true positive)

Not a liar miss (false negative)

In reality, X is not a liar

Test indicates Result is

Liar false positive

Not a liar correct rejection (true negative)

Possible outcomes of a test (contingency table)

Results

Could be due to telescoping and not intentional deception.

False neg

False pos

False neg

False pos

Corrections appear in blue

11/24/16

10

Things to notice about the 415 expert judgments:

• overall rate of deception is low: 36/415 = 8.7%• tests indicate non-deception most of the time: 306/415 = 74%• truly deceptive responses are judged non-deceptive most of the time (29/36) = 81%• truly non-deceptive responses are judged deceptive nearly 27% of the time.

Results

Sensitivity: ability of the test to correctly detect deception

Specificity*: ability of the test to correctly identify non-deceptive speakers

*refers to whether the test specifically distinguishes deceptive stress from other types of stress

11/24/16

11

Results

Sensitivity: ability of the test to correctly detect deception

=𝑡𝑜𝑡𝑎𝑙ℎ𝑖𝑡𝑠(𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒𝑠)

𝑡𝑜𝑡𝑎𝑙ℎ𝑖𝑡𝑠 + 𝑡𝑜𝑡𝑎𝑙𝑚𝑖𝑠𝑠𝑒𝑠(𝑓𝑎𝑙𝑠𝑒𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒𝑠)

Results

Specificity*: ability of the test to correctly identify non-deceptive speakers

=𝑡𝑜𝑡𝑎𝑙𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑟𝑒𝑗𝑒𝑐𝑡𝑖𝑜𝑛𝑠

𝑡𝑜𝑡𝑎𝑙𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑟𝑒𝑗𝑒𝑐𝑡𝑖𝑜𝑛𝑠 + 𝑡𝑜𝑡𝑎𝑙𝑓𝑎𝑙𝑠𝑒𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒𝑠

11/24/16

12

Sensitivity and Specificity

are commonly reported for medical tests, but alone they do not provide enough info.

Likelihood Ratio

combines both positive and negative predictive value

How big does LR have to be in order to count as compelling evidence?

11/24/16

13

Medical Interpretation of Likelihood Ratios

Computation of Likelihood Ratios

+LR = sensitivity1− specificity