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Math Help - Demographic Data, Percentages and Projections Help Needed!

  1. #1
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    Demographic Data, Percentages and Projections Help Needed!

    Hi,

    I'm trying to create projections on a user survey.

    Basically, in the question, there are 1000 computer users in a school:

    Age range | No of children

    10-12 200
    12-14 200
    14-16 400
    16-18 200


    The school is trying to work out how many Macs and PCs they should buy for next year.

    A sample of 100 users reveals some like windows some like MACs etc, such as:

    Age Range | No of children | Like MACs | Like PCs | Not Bothered

    10-12 20 5 10 5
    12-14 20 3 12 5
    14-16 40 10 7 3
    16-18 20 12 7 1

    I calculated the % for each age range for each like mac or like pc and created a new table with the projections included. In the table, I totalled the number of people who would like macs and the number of people who would like windows using these projections.

    However, if I use the sample above by taking the totals of like macs and like pcs and working out the percentage regardless of age range

    e.g.

    Total like Macs = 30
    % of sample who like macs = 30/1 = 30%
    Total like PCs = 36
    % of sample who like pcs = 36/1 = 36%

    The numbers come out differently.

    I'm confused. I'm not sure whether to work out each individual age group projections then add up the total....OR....use the method above i.e. sample has 30% who like MACs therefore 300 macs are need

    Please help !
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  2. #2
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    Quote Originally Posted by MrGreedy View Post

    Age Range | No of children | Like MACs | Like PCs | Not Bothered

    10-12 20 5 10 5
    12-14 20 3 12 5
    14-16 40 10 7 3
    16-18 20 12 7 1
    The third line of this table does not sum to 40. Should this be 20?

    Would that conflict with your sample of 100?
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  3. #3
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    I have attached a sheet to this post, it may help with your problem.
    Attached Files Attached Files
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  4. #4
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    Thanks for the sheet.

    Mistake. I should have put:

    10-12 20 5 10 5
    12-14 20 3 12 5
    14-16 40 30 7 3
    16-18 20 12 7 1

    30 instead of 10

    I think I understand it a little better...basically the first method should be correct as it takes into account the distribution of people in differing age classes...the second method uses the total without taking into account the differing sizes of age class.

    A clearer explanation would be nice.
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  5. #5
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    The table in the sheet provided shows the preferred computer type per age group and overall.

    If the school buying the computers had an unlimited budget for purchasing then consider the ratios within each age group and purchase accordingly.

    If the budget only allowed one type of computer to be purchased then the ratios in the total should be considered.
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  6. #6
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    I see....I completed your excel spreadsheet and uploaded.

    Thanks for all your help.

    The school has infinite purchasing power so using the per age group calculations will do.

    The sheet I have included has two different methods...one using each age group and then totalling the projected based on the survey...the second..using the survey as a whole and calculating the projected totals of MACs or PCs needed.

    The first method is accurate..but I'm trying to make sense in my head why the second one is off. It must be the distribution of the number of children in each age range that pushes it off.
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  7. #7
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    Quote Originally Posted by MrGreedy View Post
    It must be the distribution of the number of children in each age range that pushes it off.
    Correct.
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