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NBME 23 Answers

nbme23/Block 1/Question#13

A researcher is asked to prospectively investigate ...

Prevalence: above the standard; Incidence: above the standard

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 +3  upvote downvote
submitted by aladar50(16),

So there’s 100 residents, and the prevalence after 2 years is =10 at the beginning, +5 in the first year, +10 second year, and -3 that healed, for a total prevalence of 22 residents or 22/100=22 percent. Thus, prevalence = above the standard. For incidence, it’s 15 new cases out of 90 residents over the 2 years (100 total residents – 10 that already had ulcers), or 15 new ulcers per 180 patient⋅years. This would be 83.3 new ulcers per 1000 patient⋅years if you extrapolated it out -- basically (1000/180) * 15 -- thus, incidence = above the standard.

zelderonmorningstar  Okay I feel like an idiot cause I thought: Above the Standard = Doing a good job keeping old people from getting ulcers. Thumbs up. Below the Standard = I wouldn’t let my worst enemy into your ulcer ridden elder abuse shack. +6  
aladar50  @zelderon Ohh damn. I could totally see how one could view the answer choices that way. I think it is important to read how they are phrased - they are asking if the center is above THE standard or below THE standard. The “standard” is an arbitrary set point, and the results of the study are either above or below that cut off. Maybe if it was “above/below standards” that would work. Also, being above the standard could either be a good thing or bad thing. If say you were talking about qualifying for a competition and you have to do 50 push ups in a minute, then being above=good and below=bad. In this case, having more ulcers than the standard = bad. +1  
saynomore  @aladar Thank you!!! but how did you get the 15 new ulcers per 180 patientâ‹…years? I mean I understand the 15 part, but not the second part ... hence why I messed this up, lol :| +  
aladar50  @saysomore Because the study is looking at 100 residents over a period of 2 years. Since 10 already had the disease at the start, when looking at incidence you only include the subjects that have /the potential/ of developing the disease, so 90 patients over 2 years. This would be 90 patientâ‹…years per year, or a total of 180 patientâ‹…years over the course of the study. +2  
sympathetikey  @zelderonmorningstar I thought the same exact thing. Had the right logic, but then just put the backwards answer. +1  
kai  I wonder if they chose this wording on purpose just to fuck with us or if this was accidental. My guess is there's some evil doctor twirling his thumbs somewhere thinking you guys are below the standard. +4  

 +2  upvote downvote
submitted by keshvi(4),

i counted both the prevalence and incidence using patient - years. Is it incorrect to use patient years for prevalence?