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Median of grouped data!

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    Im doing the AQA S1 test. Median of grouped data is really annoying me.

    I have a TI-83 which calculates median, but when entering data I can only enter in midpoints as x values.

    Does anyone know how to enter in groups e.g. 10-12 instead of midpoint- 11.

    Much appreciate any help!
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    why do you need a calculator?
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    (Original post by Biffy Clyro)
    Im doing the AQA S1 test. Median of grouped data is really annoying me.

    I have a TI-83 which calculates median, but when entering data I can only enter in midpoints as x values.

    Does anyone know how to enter in groups e.g. 10-12 instead of midpoint- 11.

    Much appreciate any help!
    Erm for grouped gata you have to use the midpoints to find the median. It isn't too difficult to work out the midpoints yourself is it? And then plug them in.

    Even better do it without a grphical calculator through the standard interpolation method.
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    The examples in my book are rubbish.

    Ok, try this question:

    Time to complete a puzzle___ Frequency
    20-39_____________________6
    40-49_____________________8
    50-54_____________________7
    55-59_____________________ 5
    60-99_____________________ 9
    100+ _____________________ 5

    Total: 40

    Find the estimate of the median.

    Cheers

    --------------

    I know you can use the midpoints, but that does not give the exact estimate of the median.

    For instance, the question above using midpoints gives 52. That is not the answer.
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    (Original post by Biffy Clyro)
    The examples in my book are rubbish.

    Ok, try this question:

    Time to complete a puzzle___ Frequency
    20-39_____________________6
    40-49_____________________8
    50-54_____________________7
    55-59_____________________ 5
    60-99_____________________ 9
    100+ _____________________ 5

    Total: 40

    Find the estimate of the median.

    Cheers

    --------------

    I know you can use the midpoints, but that does not give the exact estimate of the median.

    For instance, the question above using midpoints gives 52. That is not the answer.
    That's because you've worked it out wrong....

    I got 54.1, it might be wrong if ive made a mistake
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    hmmm.. well that isnt the right answer either apparently.

    I can be sure that upon entering the right details: Midpoints are: 29.5, 44.5, 52, 57, 79.5, anything above 100. I get 52.0 as the median.. but its wrong.

    Very confused
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    (Original post by rpotter)
    That's because you've worked it out wrong....

    I got 54.1, it might be wrong if ive made a mistake
    I got 54.1 as well...
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    I get 53.78, probably because the above two are using 0.5(n+1) rather than just 0.5n, although my stats book says you should use 0.5n for the median of grouped data. If your answer book disagrees, it's probably wrong.
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    53.78 is correct!

    I cant seem to get that value though! If you could summarise your working quickly for me I would be extremely grateful.

    I really don't know how I keep getting it wrong, Im supposed to be good at Maths :rolleyes:
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    Look up interpolation in grouped data. Basically, the formula is:

    median = b + ((1/2*n - f)/f[c]) * c

    where b = the lower class boundary of the median class, f is the sum of the frequencies below b, f[c] is the frequency of the actual median class, and c is the width of the median class (n is the total frequency).

    so in this case,

    median = 49.5 + ((20-14)/7)*5 = 53.78
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    Thank You sooooo much!

    That is perfect..! My stupid book's explanation was very poor.
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    (Original post by skidjzu)
    Look up interpolation in grouped data. Basically, the formula is:

    median = b + ((1/2*n - f)/f[c]) * c

    where b = the lower class boundary of the median class, f is the sum of the frequencies below b, f[c] is the frequency of the actual median class, and c is the width of the median class (n is the total frequency).

    so in this case,

    median = 49.5 + ((20-14)/7)*5 = 53.78
    hmm, never seen that before, when I did S1 always did it a different way and got it correct, might be different boards i spose

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