1. Sensitivity and Specificity Part II – Computations and Examples

2. This video is designed to accompany pages 81-94 in Making Sense of Uncertainty Activities for Teaching Statistical Reasoning Van-Griner Publishing Company

3. ImPACT Example Concern about concussions in young athletes is at an all- time high. A variety of screening tests are available for assessing concussion status, some are to be used on the field; others take more time. One such test is the “ImPACT” test, which stands for “Immediate Post- Concussion Assessment and Cognitive Testing.” How well does ImPACT perform as a screening test?

4. ImPACT Example Rule: Beyond a certain level on the ImPACT scale you are said to be concussed. Actual Status ImPACT Prediction Athlete was, in reality, not concussed Athlete was, in reality, concussedTotals ImPACT said “not concussed” 591372 ImPACT said “concussed”759 66 Totals6672138 “Sensitivity and specificity of the ImPACT Test Battery for concussion in athletes.” Archives of Clinical Neuropsychology, 2005 FPR = 7/66 = 0.11 or 11% FNR = 13/72 = 0.18 or 18% Specificity = 89% Sensitivity = 82%

5. Beck Inventory Example The original BDI, first published in 1961, consisted of twenty-one questions about how the subject has been feeling in the last week. A typical question is below. Choose the most appropriate response (0) I do not feel sad. (1) I feel sad. (2) I am sad all the time and I can't snap out of it. (3) I am so sad or unhappy that I can't stand it. How well does Beck perform as a screening test?

6. Beck Inventory Example Rule: If your total score on the Beck Inventory is 10 or greater then you will be categorized as clinically depressed. Actual Status as determined by Diagnostic and Statistical Manual of Mental Disorders IV Beck Inventory Indication Patient was, in reality, not depressed Patient was, in reality, depressedTotals Beck said “not depressed” 66571 Beck said “depressed”12 24 Totals781795 “Sensitivity and Specificity of Depression Questionnaires in a College-Age Sample.” Journal of Genetic Psychology, 2008, 169(3), 281–288 FPR = 12/78 = 0.14 or 14% FNR = 5/17 = 0.29 or 29% Specificity = 85% Sensitivity = 71%

7. Field Sobriety Testing There are three common testing procedures implemented during an FST: horizontal gaze nystagmus (HGN), one-leg stand (OLS), and walk and turn (WAT). Data from 296 subjects who participated in the NHTSA’s 1998 San Diego field sobriety test validation study, ordered by total scores on these three FST procedures, are available. Let’s set a cutoff for the field rule and assess how well the rule is working as a screening test for intoxication.

8. Rules For The Test: assume that a 4 or above on the FST will tag a participant as drunk. Concerning The Truth: assume that a BAC of 0.04% or greater means a participant is legally drunk. There were 267 study participants with BACs of 0.04% or greater.

9. Task at Hand Using Total FST >=4 to Designate “Drunk” FST Decision Actual BAC Totals < 0.04 (Sober)>= 0.04 (Drunk) Sober Drunk Totals29267296 Fill out this table!

10. Field Sobriety Testing CaseHGNOLSWATTotal FSTActual BAC 22900110 25400110.02 66 1 10.067 14220020.005 21720020.03 1912 20.034 18220020.038 1092 20.04 25920020.04 19901120.048 11320020.05 222 20.06 672 130.022 14503030.03 5320130.032 1520130.04 28720130.04 8921030.05 12301230.05 25821030.053 3520240 1121140.01 2474 40.016 620240.02 29420240.02 23120240.03 7421140.04 21421140.04 584 40.05 1421140.058 3440040.058 1220240.06 21121140.06 23240040.06 29322040.06 13021140.07 2974 40.08 27120240.1 119 2240.121 Using Total FST >=4 to Designate “Drunk” FST Decision Actual BAC Totals < 0.04>= 0.04 Sober Drunk Totals29267296 20 911 27 6 20 256 FPR = 20/29 or 69% Specificity = 9/29 or 31% FNR = 11/267 or 4% Sensitivity = 256/267 or 96%

11. What If? Change the rule: assume that a 2 or above on the FST will tag a participant as drunk. Changes that occur: FPR will surely go up and FNR will surely go down. So the point is: how well a screening test performs is directly related to the cutoff it uses to identify a “positive.”

12. One-Sentence Reflection Simple fractions are used to compute sensitivity and specificity in many situations where both test results and the truth are arrayed in 2x2 tables.