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Thread: dealing with unequal sample sizes... can anyone help please?

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    dealing with unequal sample sizes... can anyone help please?




    Hello,
    I am comparing men and women on outcomes of their diagnostic assessment (autism). there are approx 600 men and 200 women. I have used t-tests adn chi-squared analyses to compare outcomes, and then a multivariate ANOVA with sex and diagnostic outcome (also unequal sample sizes) as fixed factors to compare scores on individual symptom measures. The data is normally distributed, but I donīt know whether the unequal sample sizes are problematic here.
    a colleague suggested using weighted means in SPSS - if I choose the 'weighted means - weight by sex' option in SPSS it seems to simply double the sample size of the women group, so teh groups are still not actually even (600 : 400). Maybe it is more complex than this, but I don't really understand if this is a sensibe thing to do, and if it is how do I report the N values for these analyses?
    any advice at all would be greatly appreciated!
    Thanks - ellie

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    Re: dealing with unequal sample sizes... can anyone help please?

    The data is normally distributed,
    Certainly not, but with n=800 it need't be.
    I donīt know whether the unequal sample sizes are problematic here. a colleague suggested using weighted means in SPSS
    Hm. You don't know whether there is a problem at all, or what
    kind of problem, but nevertheless you start fixing it instantly.

    AFAICS there is no basic problem arising from unequal group
    sizes in your study, if you want to perfom significance tests
    (by the way, did you mean multivariate ANOVA or multifactorial
    ANOVA?). Or, do you have a research question which perhaps
    might require equal groups?

    With kind regards

    K.

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    Re: dealing with unequal sample sizes... can anyone help please?

    Dear Karabiner,

    Thanks for taking the time to comment on this.

    The research questions I wanted to answer were:
    1) Is there a difference in the proportion of males and females who receive an autism diagnosis (I used chi-squared here)
    2) Within the autism group, are there sex differences on scores of diagnostic instruments (I used t-tests here)
    3) Do sex differences on scores of diagnostic instruments vary according to subdiagnostic group (there are 2 subdiagnostic groups). (I used a multivariate ANOVA here).

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    Re: dealing with unequal sample sizes... can anyone help please?

    So there's no need for weighting. If you look into your
    literature/reference studies, you certainly will find
    that weighting was nowhere performed.

    With kind regards

    K.

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    Re: dealing with unequal sample sizes... can anyone help please?

    Who are those people that have done an autistic assessment?

    I would say that it might be because it was suspected that they are autistic. So they are not representative for the general population at all. (Have you ever heard or an autism investigation among people in general?) So you can not make any conclusions at all about women and men in general (or any other subgroup), no matter what statistical method you are using.

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    Re: dealing with unequal sample sizes... can anyone help please?

    I thought this might interesting although I am not knowledgable enough about ANOVA to comment on it myself.

    https://www.uvm.edu/~dhowell/StatPag...nequal-ns.html

    I think GretaGarbo's point is on whether you can generalize from your result, with any method, rather than if the results themself are invalid (which only matters if you wanted to generalize to that larger population).
    "Very few theories have been abandoned because they were found to be invalid on the basis of empirical evidence...." Spanos, 1995

  7. The Following User Says Thank You to noetsi For This Useful Post:

    ellie_w (11-18-2014)

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    Re: dealing with unequal sample sizes... can anyone help please?

    ok - thanks Karabiner.

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    Re: dealing with unequal sample sizes... can anyone help please?


    thatīs right - Iīm not trying to make any conclusions about men and women in general, just the population that these individuals were sampled from.

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