How to do Chi-square test with given distribution

Question

I see that function for Chi-distribution are given on this page: https://wiki.freepascal.org/Generating_Random_Numbers The code (comments added) is as follows (in Pascal - easily understandable):

function randomChisq(df: integer): real;      
{comment: integer df is sent to this function and a real values is returned}
begin
  if df < 1 then 
    result := NaN                            {comment: invalid df}
  else
    result := randomGamma(0, 2, 0.5 * df);   {comment: main return value}
end;

However, how can I use this function to perform a chi-square test and calculate P value for following given data:

         Affected   notAffected
groupA     55            85
groupB     255           365

Thanks for your help.

Answer 1

I don't know pascal, but here is some base R

data = c(55,85,255,365)
#Total number in sample
N = sum(data)

#Turn into a table
tbl = matrix(data, nrow = 2, byrow =T)

#Compute the sums of rows and columns
colsums = colSums(tbl)
rowsums = rowSums(tbl)


#Compute expected cell frequencies with an outer product
#Doesn't matter what an outer product does, it just computes the expected cell counts
expected =outer(rowsums, colsums/N)

#Compute the test stat
tst_stat = sum((tbl-expected)^2/expected)

chi_square_samples = rchisq(100000,1)
mean(chi_square_samples>=tst_stat)

Approximately 68% of samples from a chi square distribution with 1 degree of freedom are larger than or equal to the test statistic. So, the p value is around 0.68. We can confirm this with R's built in function for the chi-square test

> chisq.test(tbl, correct = F)

    Pearson's Chi-squared test

data:  tbl
X-squared = 0.16068, df = 1, p-value = 0.6885