Day 1: Standard Deviation Puzzles #1

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  • hackmaverick 9 years ago + 1 comment

    Could you specify whether the answer is for Sample Standard Deviation or Population Standard Deviation? Both are different answers.

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    • spgarbet 9 years ago + 1 comment

      It's the population standard deviation. Further, they don't specify if one can submit the highest value under which rounding. Turns out it's the correct value, rounded to 2 decimals.

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      • pepa_k_gencheva 9 years ago + 0 comments

        Actually, it is not rounding, but flooring (cutting after the 2nd decimal place) ...

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    • sahmad39 9 years ago + 1 comment

      I'm confused about what this problem is asking for.

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      • hhoumane 9 years ago + 0 comments

        Find a number N such that the standard deviation of [1,2,3,N] is equal to the standard deviation of [1,2,3] up to the second digit.
        Doing it algebrically wouldn't work because the two standard deviations aren't exactly equal, merely close enough to fool us up to the second digit, so there is no equation to solve.

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      • FL750 9 years ago + 2 comments

        Excel solver, Period

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        • marcoafo 9 years ago + 0 comments

          Indeed. I tried doing the math, but eventually I did something wrong in the way.

          Just a note for people that will do this, make sure you get a big startup point, because the problem asked for the largest number (I got a wrong answer due to this).

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          • vkkumar 9 years ago + 0 comments

            scipy.optimize has fsolve

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          • Parth_Vishvajit 9 years ago + 3 comments

            How to find out an answer? I mean what's the method? I found the standard deviation sqrt(2/3) for set{1,2,3}, which is obvious. but what's the calculation to find out the N? I tried the LHS=RHS methodology but i am getting a quardratic eqation which is having imaginary roots. Please guide me.. looks like i am making some huge mistake. Suggest me the methodology. Thank you

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            • yashkaf Asked to answer 9 years ago + 0 comments

              Since this is a programming competition after all, and not an algebra one, how about using a few lines of code to brute-force the answer?

              Since you only need the answer to within 0.01, you can probably write some sort of a loop that will increment numbers by 0.01 and check their standard deviations...

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              • anil_motupalli Asked to answer 9 years ago + 1 comment

                Try to re-arrange the equation to extract square of another equation of power one instead of finding the roots.

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                • Parth_Vishvajit 9 years ago + 1 comment

                  You mean to say i should bring it in the form like [(a+b)^2] right? i tried. but my equation is n^2 - 36n +484 =0. is it the right eqation? or you can guide me to right one. Thank you.

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                  • anil_motupalli 9 years ago + 0 comments

                    There is something wrong with the constant. 484 is pretty huge. Check your solution again.

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                • shashi_k 9 years ago + 1 comment

                  solve your equation here http://www.homeschoolmath.net/worksheets/equation_calculator.php

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                  • AlejandroBlanco 9 years ago + 0 comments

                    For some reason my equation returns a wrong result in the Solve tab (despite the Graph tab returns the correct one!)

                    You may use for a correct result:

                    http://www.quickmath.com/webMathematica3/quickmath/equations/solve/basic.jsp

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                • nomino 9 years ago + 0 comments

                  In the previous probelm you asked to compute the biased standard deviation, although usally (and correctly) it is used the unbiased one. What should we use in this problem?

                  EDIT: Tried the biased version and it worked.

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