Sensitivity and Specificity. Part II – Computations and Examples. This video is designed to accompany pages 81-94 in Making Sense of Uncertainty Activities for Teaching Statistical Reasoning Van- Griner Publishing Company. ImPACT Example.
Part II – Computations and Examples
Making Sense of Uncertainty
Activities for Teaching Statistical Reasoning
Van-Griner Publishing Company
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?
“Sensitivity and specificity of the ImPACT Test Battery for concussion in athletes.” Archives of Clinical Neuropsychology, 2005
FPR = 7/66 = 0.11 or 11%
Specificity = 89%
Sensitivity = 82%
FNR = 13/72 = 0.18 or 18%
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?
“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%
Specificity = 85%
Sensitivity = 71%
FNR = 5/17 = 0.29 or 29%
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.
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.
Fill out this table!
FPR = 20/29 or 69%
Specificity = 9/29 or 31%
Sensitivity = 256/267 or 96%
FNR = 11/267 or 4%
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.”
Simple fractions are used to compute sensitivity and specificity in many situations where both test results and the truth are arrayed in 2x2 tables.