[R] help please. 2 tables, which test?

[Greg Snow]

For this case I would use a permutation test.  Start by choosing some
statistic that represents your 4 students across the different grades,
some possibilities would be the sum of scores across grades and
students, or mean, or median, or ...

Compute the selected statistic for your 4 students and save that
value.  Now select 4 students at random and compute the same
statistic, repeat this a bunch of times (thousands) and compute the
statistic each time.  All those stats on the random selections
represent the distribution of the statistic under the null hypothesis
that your 4 students were randomly chosen (vs. chosen based on
something that is related to the grade).  Now you just compare the
stat on the original 4 students to the distribution (if you need a
specific p-value it is just the proportion of the random stats that
are as or more extreme as your original 4).

On Sat, Mar 10, 2012 at 4:04 AM, aoife doherty <aaral.singh at gmail.com> wrote:
> Thank you for the replies.
> So what my test wants to do is this:
>
> I have a big matrix, 30 rows (students in a class) X 50 columns (students
> grades for the year).
> An example of the matrix is as such:
>
>
>                     grade1       grade2        grade3     .....  grade 50
> student 1
> student 2***
> student 3
> student 4***
> student 5***
> student 6
> .
> .
> .
> .
> .
> student 30***
>
> As you can see, four students (students 2,4,5 and 30) have stars beside
> their name. I have chosen these students based on a particular
> characteristic that they all share.I then pulled these students out to make
> a new table:
>
>                     grade1          grade2         grade3 ....... grade 50
>
> student 2
> student 4
> student 5
> student 30
>
>
> and what i want to see is basically is there any difference between the
> grades this particular set of students(i.e. student 2,4,5 and 30) got, and
> the class as a whole?
>
> So my null hypothesis is that there is no difference between this set of
> students grades, and what you would expect from the class as a whole.
>
> Aaral
>
>
>
>
>
>
> On Sat, Mar 10, 2012 at 12:18 AM, Greg Snow <538280 at gmail.com> wrote:
>>
>> Just what null hypothesis are you trying to test or what question are
>> you trying to answer by comparing 2 matrices of different size?
>>
>> I think you need to figure out what your real question is before
>> worrying about which test might work on it.
>>
>> Trying to get your data to fit a given test rather than finding the
>> appropriate test or other procedure to answer your question is like
>> buying a new suit then having plastic surgery to make you fit the suit
>> rather than having the tailor modify the suit to fit you.
>>
>> If you can give us more information about what your question is we
>> have a better chance of actually helping you.
>>
>> On Fri, Mar 9, 2012 at 9:46 AM, aoife doherty <aaral.singh at gmail.com>
>> wrote:
>> >
>> > Thank you. Can the chi-squared test compare two matrices that are not
>> > the
>> > same size, eg if matrix 1 is a 2 X 4 table, and matrix 2 is a 3 X 5
>> > matrix?
>> >
>> >
>> >
>> > On Fri, Mar 9, 2012 at 4:37 PM, Greg Snow <538280 at gmail.com> wrote:
>> >>
>> >> The chi-squared test is one option (and seems reasonable to me if it
>> >> the the proportions/patterns that you want to test).  One way to do
>> >> the test is to combine your 2 matrices into a 3 dimensional array (the
>> >> abind package may help here) and test using the loglin function.
>> >>
>> >> On Thu, Mar 8, 2012 at 5:46 AM, aaral singh <aaral.singh at gmail.com>
>> >> wrote:
>> >> > Hi.Please help if someone can.
>> >> >
>> >> > Problem:
>> >> > I have 2 matrices
>> >> >
>> >> > Eg
>> >> >
>> >> > matrix 1:
>> >> >                Freq  None  Some
>> >> >  Heavy    3        2          5
>> >> >  Never    8       13         8
>> >> >  Occas    1        4          4
>> >> >  Regul     9        5         7
>> >> >
>> >> > matrix 2:
>> >> >                  Freq     None     Some
>> >> >  Heavy        7          1             3
>> >> >  Never      87         18          84
>> >> >  Occas      12           3            4
>> >> >  Regul        9            1            7
>> >> >
>> >> >
>> >> > I want to see if matrix 1 is significantly different from matrix 2. I
>> >> > consider using a chi-squared test. Is this appropriate?
>> >> > Could anyone advise?
>> >> > Many thank you.
>> >> > Aaral Singh
>> >> >
>> >> > --
>> >> > View this message in context:
>> >> >
>> >> > http://r.789695.n4.nabble.com/help-please-2-tables-which-test-tp4456312p4456312.html
>> >> > Sent from the R help mailing list archive at Nabble.com.
>> >> >
>> >> > ______________________________________________
>> >> > R-help at r-project.org mailing list
>> >> > https://stat.ethz.ch/mailman/listinfo/r-help
>> >> > PLEASE do read the posting guide
>> >> > http://www.R-project.org/posting-guide.html
>> >> > and provide commented, minimal, self-contained, reproducible code.
>> >>
>> >>
>> >>
>> >> --
>> >> Gregory (Greg) L. Snow Ph.D.
>> >> 538280 at gmail.com
>> >>
>> >> ______________________________________________
>> >> R-help at r-project.org mailing list
>> >> https://stat.ethz.ch/mailman/listinfo/r-help
>> >> PLEASE do read the posting guide
>> >> http://www.R-project.org/posting-guide.html
>> >> and provide commented, minimal, self-contained, reproducible code.
>> >
>> >
>>
>>
>>
>> --
>> Gregory (Greg) L. Snow Ph.D.
>> 538280 at gmail.com
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
>



-- 
Gregory (Greg) L. Snow Ph.D.
538280 at gmail.com