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Old 2013-02-23, 03:59   #1
only_human
 
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Default Wreck your brain on this...

Bledar H on g+ says "Wreck your brain on this...":
Quote:
If you were to choose an answer to this question at random, what is the chance you will be correct?
  1. 25 %
  2. 50 %
  3. 60 %
  4. 25 %
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Old 2013-02-23, 04:39   #2
firejuggler
 
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25% obviously, since only one answer is correct.
the fact that there is the good answer 2 time doesn't affect the probability.
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Old 2013-02-23, 05:11   #3
only_human
 
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Quote:
Originally Posted by firejuggler View Post
25% obviously, since only one answer is correct.
Multiple choice questions often have multiple correct answers. Sometimes they also include the choices: none of the above & all of the above. I parse this as: If you randomly pick an answer from the set {A,B,C,D}, how likely will your answer be scored as correct? Perhaps checked by a machine or a template (assuming that it is configured as intended).

Last fiddled with by only_human on 2013-02-23 at 05:25
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Old 2013-02-23, 05:31   #4
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It's a disguised Jourdain's postcard paradox.
Quote:
Originally Posted by wiki
Suppose there is a card with statements printed on both sides:
Front:The sentence on the other side of this card is TRUE.
Back:The sentence on the other side of this card is FALSE.

Trying to assign a truth value to either of them leads to a paradox.
In this OP formulation:
If you assign a truth value to the correctness of answer 25%, the answer becomes 50%.
If you assign a truth value to the correctness of answer 50%, the answer becomes 25%.

Last fiddled with by Batalov on 2013-02-23 at 05:33
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Old 2013-02-23, 05:45   #5
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Ding! Ding! Ding! Sergey is right. Congrats. I was tempted to delay responding to collect some fun guesses. Follow the link above to see some amusing comments.

Last fiddled with by only_human on 2013-02-23 at 05:57
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Old 2013-02-23, 06:25   #6
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Yesterday I answered an online customer satisfaction survey for DelTaco. Most of the answers involved picking a number from 1 to 10 to select graduations from least satisfied to most satisfied. One question was something like "Select 5 if you are reading this." It was obviously chosen to smoke out the people that gush praise or scorn by marking answers down a column without even reading the questions.

I had a teacher (FORTRAN) that would round out his tests by added a couple of whimsical questions to make the point total add up to 100. One time a question said "Check this box for 2 points." Another time he was sick and had another person hand out the tests. I wasn't officially enrolled in the class and wasn't present on the test day but I was there when he handed out the results. His whimsical question that time was "How many people are in a string quartet?" Unfortunately, most of the class answered "two." I mention this because his whimsy became an inadvertent smoke-out too. He was furious (suspecting answer copying en masse) and told them that he would like to flunk the class but couldn't take that to administration. One of the things I liked about him was that during lectures he would sometimes pause and pose a question to the students letting them informally just shout back answers; he would write all the answers on the board. It was amusing to see the different things people would say -- a lot like brainstorming but without the brain.

Last fiddled with by only_human on 2013-02-23 at 06:47
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Old 2013-02-23, 10:59   #7
xilman
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Quote:
Originally Posted by only_human View Post
I had a teacher (FORTRAN) that would round out his tests by added a couple of whimsical questions to make the point total add up to 100. One time a question said "Check this box for 2 points." Another time he was sick and had another person hand out the tests. I wasn't officially enrolled in the class and wasn't present on the test day but I was there when he handed out the results. His whimsical question that time was "How many people are in a string quartet?" Unfortunately, most of the class answered "two." I mention this because his whimsy became an inadvertent smoke-out too.
My first practical lab in inorganic chemistry was a fairly straightforward quantitative analysis of a solution of a metallic ion I forget the details but we had to precipitate an insoluble salt by adding some reagent or other, filter it, dry it and weigh it. Tedious, like much of practical chemistry IME, but not especially difficult.

A week after the results were all handed in, a histogram plot was presented showing number of students finding a particular concentration against concentration of metal ion. It was double peaked. The major peak was centred on the true concentration as made up by the lab staff. The minor peak was centred on the true concentration of there previous year's solution.

The lab ran this experiment for years and always found the same result. It tells you something interesting about the psychology of first and second year undergraduate chemists. First, that neophytes think they can get away with cheating. Second, that second-year students either weren't paying attention when they were first-years, or that they have a perverse sense of humour. I favour the latter explanation because it fits in well with other observations.



Paul
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Old 2013-02-24, 21:34   #8
ewmayer
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I suspect I accounted for a "third peak" in one of my freshman inorganic-chem labs ... one of the trickier labs toward the end of the term involved some multistep process deliberately designed to make it more difficult than necessary to produce a simple compound - copper sulfate, those famously bright blue crystals (I refer to the common pentahydrate CuSO4·5H2O form, as opposed to the anhydrous), in order to test our accumulated laboratory skills. I and my lab partner were going great but fubared some step toward the very end, which left us with something far different than a simple solution of CuSO4 read for drying and weighing. In desperation I sneaked over to the part of the lab where they stored the common reagents and such, grabbed a handful of reagent-grade CuSO4 and sneaked it back to our lab bench for quick dissolution. Our resulting yield was even higher than the "best expected".

Think of it as the chem-lab equivalent of a Rosie Ruiz.
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