ELI5 - When filling multiple choice bubbles at random why only go with 1 letter?

Submitted by Stellar_Panda t3_10q2e67 in explainlikeimfive

It's a tip I hear often and I don't quite understand. If the time is nearly up and you have to fill in answers for multiple choice, A,B,C,D for example. Why would you only go with 'A'? You should get around 25% of the answers correct but no proper test is ever going to have 'A' as correct answer many times in a row.

Would it not be marginally better to just pick at random? Why not?

Edit: I understand real world is rarely truly random, but is my thinking here correct given answers are randomly distributed?

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Lithuim t1_j6ne7su wrote

If the answers are truly distributed randomly, then any random selection of letters would produce roughly the same score.

Whether they’re actually random or not would depend on who wrote the test.

Whether you’d get a better score by guessing or not would depend on the way the test is graded - many standardized tests subtract 1/4th of a point for wrong answers so that a random guesser will get a zero.

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breckenridgeback t1_j6nhre4 wrote

That's a pretty big "if", though. A lot of tests are written by hand and bias away from long answer streaks and first/last answer choices.

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AcusTwinhammer t1_j6o8gdy wrote

Yes--back when I was in high school in the late 80s, one of my teachers also taught a SAT prep class, and at the time they were teaching a strategy where if you had no idea and just had to guess, then avoid ACEs and doubles. A was too easy, E too hard, C too obvious, and test creators don't like answer streaks. I would assume test creation methodology has changed for the SAT in 30 years, but it still may be helpful for any human-created multiple choice tests out there.

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cyklone117 t1_j6olruy wrote

I had a friend in high school who used the magician's run, a.k.a. ABBACADABBA

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breckenridgeback t1_j6olqhr wrote

It would surprise me if that were accurate for the SAT even back then.

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AcusTwinhammer t1_j6or182 wrote

I didn't take the class, it was just a quick summary he gave of part of his class, but if humans are selecting the answers, or even just reviewing and editing the answers, there is going to be some sort of bias--"that's too many Cs in a row," or "there's too many As in this section." Whether or not one specific "SAT tips!" advice or another was actually better, I don't know.

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Turnip45 t1_j6nro2v wrote

Provided the scattering is random, or at least random enough given the sample size, then there’s not actually a statistical advantage to picking one letter vs picking a scatter.

Only slight advantage at just filling in a single letter is it saves time guessing if you’re real short on time.

If you ever do this though make sure you understand all the grading rules, some tests deduct points for incorrect answers.

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Faleya t1_j6nei0i wrote

afair one should select "C" in that situation as it is slightly more common than the other options usually.

the idea is that this way you "make sure" that some of your answers will be correct, while if you select answers at random you might statistically get as many or more correct, you can also just miss the correct answer each time. but it is very very unlikely that a test is designed in a way that "C" would never be the correct answer for 15 consecutive questions, for example.

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Stellar_Panda OP t1_j6ogo50 wrote

I understand this and it makes sense but is there no marginable percentage increase if do, say B and C for my answer and go through and randomly select a few B's along side majority C's. Given you have reasonable expectation that the correct answers won't be a straight line of ALL C's. Would this not give any slight increase in score?

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Faleya t1_j6oqx7w wrote

sure, unless you somehow chose B on all of the answers where C is correct and C on those where A, B or D is correct.

so yeah you have a chance to get a few more right, but also to have a few more (or all of them) wrong.

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TheUncannyFoxhound t1_j6nengx wrote

I don't know the stats specifically, but the answers generally weren't placed at random. In my head, that would likely make the first and last answer the least likely (outside of the last answer being all or none of the above). Thus, the middle answers are more likely to be selected for the correct answer. "C" then naturally allows you to read more answers. My unsupported two cents anyways.

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DiamondIceNS t1_j6oc7ja wrote

As others have stated, if the exam has its answers distributed truly randomly (or at least sufficiently randomly, i.e. by a computer), and if all of the answers had their choice selection decided independently, then your guesses will not matter at all. You gain no statistical advantage by any strategy. You are simply rolling an X-sided die Y number of times, where X is how many choices each question has and Y is how many questions the exam has.

The adage that you should select the same letter multiple times in a row to get an edge stems from two things, one of which is completely unrelated (and may not apply) and the other only holds if the assumptions we made aren't true.

The first is about speed. If you mark every question with something, you are statistically expected to get at least a score of 1/X on all those questions you marked. So if you anticipate the possibility that you might not even finish your exam, if you have them all pre-marked (switching answers to the correct ones as you read your way through the exam for real), then you might get some extra scoring for your guesses on questions you may have otherwise marked blank. This only works, of course, if the exam you are taking doesn't penalize incorrect answers. An exam that marks non-answers and wrong answers the same benefits from guesses; an exam that subtracts points for wrong answers and does nothing for non-answers punishes guesses.

The second is that there is some evidence that in multiple-choice exams with answer keys arranged by humans one letter is statistically more likely to be the answer for any given question. In the common four-choice arrangement with A, B, C, and D, that letter tends to be C. So, provided your exam was written by a human, and the exam doesn't penalize guessing, answering all questions with C has a statistical advantage over random guessing.

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Stellar_Panda OP t1_j6oh8mo wrote

Words are getting a bit too big for five year old. Lol Thank you for your response. But still I ask: Is there no marginable percentage increase if I do, say B and C for my answers and go through and randomly select a few B's along side majority C's? Given you have reasonable expectation that the correct answers won't be a straight line of ALL C's. Would this not give any slight increase in score? Given answers are distributed randomly.

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DiamondIceNS t1_j6oy6v3 wrote

Let me ask a counter question:

You are playing a game where someone rolls a 6-sided die over and over, and each time you have to guess which number it lands on before it is rolled. If you guess correctly, you win.

You happen to know that the die is loaded. It's not a 1/6 chance for each number to show up. The side numbered "4" is slightly more likely to occur than all of the others.

Knowing that, why would you ever guess anything other than "4"? You're not going to win every time, but it's always your best shot.

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Stellar_Panda OP t1_j6pe0pe wrote

Okay, why is selecting a straight line of 'C's better than filling in with no rhyme or reason (like the test probably would have looked like if you actually took it?) I think this is what my question boils down to.

If everything is 1/4 why would always choosing the same answer increase your chances? (Ignoring the whole 'C' is actually a better guess findings.) It doesn't right?-

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DiamondIceNS t1_j6pf6rj wrote

If the test is written by a person, C is most likely. Because the answers are (probably) not 1/4 each.

If the test was shuffled by a machine, and the answers are perfect 1/4 chance, then no strategy is better than any other. Picking straight C is just as effective as picking C most of the time and picking B sometimes, and just as effective as picking with no pattern at all.

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Stellar_Panda OP t1_j6pfl2l wrote

If machine wouldn't having a pattern hurt you? If random, try to be random. Would this not offer some kind of marginal benefit? Because very low probability of answer always being C, etc? I guess not..?

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Stellar_Panda OP t1_j6pdmk1 wrote

Okay, why is selecting a straight line of 'C's better than filling in with no rhyme or reason like the test would have looked if you actually took it? I think this is what my question boils down to.

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heidismiles t1_j6nnei8 wrote

If you guess randomly (CBADBA or whatever), it's possible to be unlucky enough to get every guess wrong.

If you guess AAAAA, you basically guarantee that about 20% of them are correct.

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Turnip45 t1_j6nt32o wrote

Not really - there’s equal chance that there’s a long string of zero hits on any given letter as there is a chance of of weaving a thread and missing all correct answers with the same number of random guesses.

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Stellar_Panda OP t1_j6oh3s3 wrote

Is there no marginable percentage increase if I do, say B and C for my answers and go through and randomly select a few B's along side majority C's? Given you have reasonable expectation that the correct answers won't be a straight line of ALL C's. Would this not give any slight increase in score? Given answers are distributed randomly.

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