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just t
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2009/07/15 20:25:21 (permalink)

For the maths/stats people (Mean and STDEV)

Long shot but here goes.


Is standard deviation evenly spread about the mean?

I.e.

if i have a mean of 20 and a standard deviation of 5

would the 5 be spread around the 20 i.e. 17.5-22.5?

or does it not work like this?

Cheers

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just t provided 5 reward points for the best answer

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    Blue_Lagoon3000
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:28:53 (permalink)
    20x is the derivative of x^20. Its integrate is x^20+c


    Lol i have no idea what that actually means
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    john_cappa
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:30:46 (permalink)
    To a degree it is but it is not as perfect as you have put it. The standard D measures the variability of the data.

    i.e a low SD means that all the values are relatively close to the mean.
     
    A large SD means there is greater difference in the data
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    just t
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:32:38 (permalink)
    DOH, im trying to ploy Standard deviation on a graph,

    not sure how to do it if the deviation is not spread evenly around the mean :S

    :(

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    john_cappa
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:35:03 (permalink)
    just t

    would the 5 be spread around the 20 i.e. 17.5-22.5?

    Cheers


    for the above example the values would typically fall between 15-25 not 17.5-22.5
     
    Again the key word is typical! not every value may fall in this range
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    just t
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:36:57 (permalink)
    ahh, my graphs gona be huge and unreadable. :~|

    and thats still assuming the STEV to be evenly spread around the mean :S

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    JK2
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:38:02 (permalink)
    No.

    If you have a mean of 20 and a Standard Deviation of 5, you're essentially saying that most of your sample fall within 5 points of the mean (so between 15 and 25).

    It varies if you have a non-standard distribution.

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    just t
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:40:08 (permalink)
    infact,

    my mean is 84, my stdev is 22.

    its obviously not evenly spread around the mean or i would have a value >100% if i plot the stdev like that :S

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    john_cappa
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:40:39 (permalink)
    just t

    ahh, my graphs gona be huge and unreadable. :~|

    and thats still assuming the STEV to be evenly spread around the mean :S


    you have me half confused now! where is john kerr! he is up to speed on this kind of thing!
     
    Whether or not is is spread evenly around the mean would depend on whether or not the data is normally distributed
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    just t
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:43:01 (permalink)
    JohnKerr2

    No.

    If you have a mean of 20 and a Standard Deviation of 5, you're essentially saying that most of your sample fall within 5 points of the mean (so between 15 and 25).

    It varies if you have a non-standard distribution.


    how does that work if the mean is say 95% with a STDEV greater than 5? say 10

    Cant be 85-105% can it?

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    JK2
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:46:17 (permalink)
    If your data is skewed (that is, the plot doesn't fall in a symmetrical curve) then means and standard deviations aren't particularly useful.

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    john_cappa
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:50:04 (permalink)
    JohnKerr2

    If your data is skewed (that is, the plot doesn't fall in a symmetrical curve) then means and standard deviations aren't particularly useful.


    if the data is skewed the median is a better description of the data as opposed to using the mean.
     
    The inter quartile range i think is then more useful than the SD also
     
     
     
    #12
    just t
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:51:23 (permalink)
    DOH,


    back to the drawing board

    thanks johnkerr

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    Blue_Lagoon3000
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:53:14 (permalink)
    The standard deviation is the common measure of statistical dispersion. A low standard deviation means most of the values will be close to the mean.
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    JK2
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:54:05 (permalink)
    just t

    DOH,


    back to the drawing board

    thanks johnkerr


    What is it you're trying to do?

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    john_cappa
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:55:21 (permalink)
    just t

    DOH,


    back to the drawing board

    thanks johnkerr


    your welcome also....considering he said nothing i hadnt already said.....
     
     
    ;(
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    Blue_Lagoon3000
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:56:32 (permalink)
    Btw that second answer was a serious one
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    just t
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:57:23 (permalink)
    john_cappa

    JohnKerr2

    If your data is skewed (that is, the plot doesn't fall in a symmetrical curve) then means and standard deviations aren't particularly useful.


    if the data is skewed the median is a better description of the data as opposed to using the mean.
     
    The inter quartile range i think is then more useful than the SD also
     
     
     


    Can really use the median.


    I have 5 sets of results comparing 3 different treatments to a control and im trying to display the mean differences from the control from each of the treatments.



    E.g. Measuring  enzyme inactivation in cabbage after cooking compared with a raw sample

                            1           2          3          4          5        Avg       STDEV
    Steamed          44        97        88        93        97        84          22
    Boiled               98        99        96       98        100       98          1
    Microwaved      97        98        97        99        100     98           1

    I have plotted the Avg (mean) on a graph, wanted the error bars to display the stdev to show any potential overlap between the steamed and other treatment.

    I have done a statistical analysis to show the significance, but it doesnt show a significant difference between the steamed and the other treatments because they are being compared with the raw which is MUCH higher and i cant really rerun the stats without the raw to show the sig dif because i got told that is not the correct protocol, so i was trying to display it graphically



    post edited by just t - 2009/07/15 20:59:43

    #18
    john_cappa
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 20:57:57 (permalink)
    Blue_Lagoon3000

    Btw that second answer was a serious one


    sorry we are no good here! we only wanted the damn 5 points!!!!!!!!
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    JK2
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    Re:For the maths/stats people (Mean and STDEV) 2009/07/15 21:00:07 (permalink)
    So you have samples of just five for each condition?

    #20
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