Friday, August 10, 2012

More vampires in psychology: Signal Detection Theory and The Vampire in the Crowd

The summer is trailing off, my grants are submitted, and my thoughts have turned to my fall courses.

I am teaching the Lab-Requirement-Filling (and therefore desperately sought out) Perception Lab.  This involves a review of Signal Detection Theory.

As an undergrad (and a grad student, and a post doc, and an assistant professor), I always found SDT SOOOOOOO Boring, no doubt thanks to material like this (from the textbook I use for the class):

"Suppose that, for an individual subject, the proportion of hits was 0.70 and the proportion of false alarms was 0.10.  Looking up 0.70 (under "p") in the table, we find a corresponding value of z = 0.52"

--St. James, D. J., Schneider, W., & Eschman, A.  (2005).  PsychMate Student Guide Version 2.0.  Psychology Software Tools, Pittsburgh.

Even though I tend to think the discussion of SDT in this text is fairly clear and straightforward, it's pretty impossible (especially as a non-expert) to read pages of that and not become completely disenfranchised.  This is too bad because, when I stop to think about it, SDT is actually AWESOME, and useful / applicable ALL THE TIME, such as in situations like these:

[ETA:  For full resolution slides, just click.  Your fancy browser will probably open a full res slide show.]

...And in all kinds of less ridiculous but hugely important ventures such as:

  • Testing for diseases
  • Finding tumors in radiology screens
  • Monitoring food in a gross "unwrapped" style food-sembly line for "abnormalities" (read, a mouse in a snickers)
  • Screening for weapons in baggage x-rays
  • Identifying counterfeit currency

Thus, I decided to present SDT to my students this year in terms of the Vampire in a Crowd parable.


Say you go to a crowded nighttime showing of Bela Lugosi's Dracula.  Despite the cheesy makeup and special effects, you become so freaked out by the movie that you are CONVINCED that the other audience members may be vampires.  You get out your stake for the spooky walk to your car after the movie.  When you hear footsteps behind you, you are faced with a problem:  should you stake the person approaching you through the heart or should you ignore them?  Assume that at some point in your journey to your morbidly parked car, each member of the audience approaches you from the shadows of the parking lot.  

It goes on to motivate the ROC in terms of the "consequences" of hits, misses, false alarms, and correct rejections in this scenario (e.g., having your blood sucked, accidentally staking a member of the Brooklyn Tabernacle Choir).

In sum, I am very sad that I don't actually get to give this lecture:  my TAs do all the actual teaching for this course, because it is so big.  Maybe I can do a guest spot!


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