Disease Detectives B/C

[Sodium_]

I was wondering, I've been doing this event for a long time and it seems I always end up studying the wrong areas/topics. For the regionals test, there were a couple topics which i studied vaguely, so I couldn't remember the answers. Also, the stuff i did study, like the Bradford Hill criteria, were of no use to me in the test. i would really like to try and get a single digit place on this event for state. What would you recommend studying, and some links and other stuff would be really helpful to me. thanks!

[hmath729]

Hey Sodium_, remember that the topic is environmental quality? Research some diseases caused by natural problems, such as drought. I already had regionals and placed 5th in DD... without studying environment. So study the environmental causes - it'll really help.

[hmath729]

Oh, and sodium, here are some links for studying.
For the CDC website on environmental health, click here. It's super helpful.
For a good practice test, click here.
Happy studying!

[Sodium_]

Thanks so much hmath! I did study environmental stuff, which was absolutely no use to me. - in our region, no matter the topic, the questions always seem to be foodborne, which doesn't make much sense, haha. Anyways, thank you for the links and everything, it helped a lot!

[hmath729]

No problem. Glad to help.

[cosine]

Is odds ratio the only thing used for case-control studies?

And the equation is ad/bc, correct? What if the denominator is 0? Is infinity a valid odds ratio? For example, take the two following tables:

Exposed to substance 1 Case patients Controls Total
Yes a = 30 b = 20 50
No c = 0 d = 5 5

Odds ratio: 150/0

Exposed to substance 2 Case patients Controls Total
Yes a = 30 b = 10 40
No c = 0 d = 15 15
Odds ratio: 450/0

In both cases, they are infinity. It does look to me that substance 2 is the more likely cause though.Am I interpreting these wrong?

[Flavorflav]

There are several methods for that. One is to add 0.5 to every cell, another is to add it to only the cell with the 0. Either way you aren't doing a pure Odds Ratio, but a Modified Maximum Likelihood Estimator of the OR.

ETA: you shouldn't see that situation arise in competition, though.

[hmath729]

We had regionals a few weeks ago and we got that exact problem - the denominator was zero. I don't get it.

[hscmom]

There are several methods for that. One is to add 0.5 to every cell, another is to add it to only the cell with the 0. Either way you aren't doing a pure Odds Ratio, but a Modified Maximum Likelihood Estimator of the OR.

ETA: you shouldn't see that situation arise in competition, though.

Our teammate (who's done this a bit longer than we have) saw it in competition (and kudos to him for knowing what to do -- he was probably 11 or 12 at the time!).

[Flavorflav]

Experts disagree on what to do in that situation, so I don't think it is a very good question for students.