[R] Getting all possible contingency tables

[Christofer Bogaso]

Thanks Ted for your correction. I was depressed thinking that I did not 
understand the theory. However now it comes as just a plotting mistake!

Thanks,

On 02 December 2012 20:47:48, (Ted Harding) wrote:
> On 02-Dec-2012 14:17:20 Christofer Bogaso wrote:
>> Thanks Ted (and other) for your suggestion. Here I have implemented
>> following:
>>
>> Tab <- matrix(c(8, 10, 12, 6), nr = 2)
>>
>> Simu_Number <- 50000
>> Tab_Simulate <- vector("list", length = Simu_Number)
>> for (i in 1:Simu_Number) {
>>           Tab_Simulate[[i]] <- matrix(rmultinom(1, sum(Tab), rep(0.25,
>> 4)), nrow = 2)   ## All Cells have equal probability
>>       }
>> Sample_ChiSq <- sapply(Tab_Simulate, function(x) {
>>                                   Statistic <-
>> sum((chisq.test(as.table(x))$observed -
>> chisq.test(as.table(x))$expected)^2/chisq.test(as.table(x))$expected)
>>                                   return(Statistic)
>>                               })
>>
>> length(Sample_ChiSq[Sample_ChiSq < qchisq(0.95, 1)])/Simu_Number
>>
>> hist(Sample_ChiSq, freq = FALSE)
>> lines(dchisq(seq(min(Sample_ChiSq), max(Sample_ChiSq), by = 0.5), 1))
>>
>>
>> However I think I am making some serious mistake as histogram did not
>> match the density curve.
>>
>> Can somebody help me where I am making mistake?
>>
>> Thanks and regards,
>> [the remainder (copies of previous posts) snipped]
>
> The reasons for the mis-match are:
>
> A: that you have put the curve in the wrong place, by not
> supplying x-coordinates to lines(), so that it plots its
> points at x = 1,2,3,4,...
>
> B: that you need to multiply the plotted density by the width
> of the histogram cells, so as to match the density curve to the
> discrete density of the histogram. It will also then look better
> when the chis-squared curve is plotted at the mid-points of the cells.
>
> Hence, try something like:
>
> hist(Sample_ChiSq, freq = FALSE, breaks=0.5*(0:40))
> x0 <- 0.25+0.5*(0:20)
> lines(x0,dchisq(x0,1))
>
> Hoping this helps,
> Ted.
>
> -------------------------------------------------
> E-Mail: (Ted Harding) <Ted.Harding at wlandres.net>
> Date: 02-Dec-2012  Time: 15:02:45
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>