6 /* This program is implemented with ideas from this web page:
8 * http://www.langsrud.com/fisher.htm
12 static double lbinom(int n, int k)
14 if (k == 0 || n == k) return 0;
15 return lgamma(n+1) - lgamma(k+1) - lgamma(n-k+1);
23 // hypergeometric distribution
24 static double hypergeo(int n11, int n1_, int n_1, int n)
26 return exp(lbinom(n1_, n11) + lbinom(n-n1_, n_1-n11) - lbinom(n, n_1));
34 // incremental version of hypergenometric distribution
35 static double hypergeo_acc(int n11, int n1_, int n_1, int n, hgacc_t *aux)
37 if (n1_ || n_1 || n) {
38 aux->n11 = n11; aux->n1_ = n1_; aux->n_1 = n_1; aux->n = n;
39 } else { // then only n11 changed; the rest fixed
40 if (n11%11 && n11 + aux->n - aux->n1_ - aux->n_1) {
41 if (n11 == aux->n11 + 1) { // incremental
42 aux->p *= (double)(aux->n1_ - aux->n11) / n11
43 * (aux->n_1 - aux->n11) / (n11 + aux->n - aux->n1_ - aux->n_1);
47 if (n11 == aux->n11 - 1) { // incremental
48 aux->p *= (double)aux->n11 / (aux->n1_ - n11)
49 * (aux->n11 + aux->n - aux->n1_ - aux->n_1) / (aux->n_1 - n11);
56 aux->p = hypergeo(aux->n11, aux->n1_, aux->n_1, aux->n);
60 double kt_fisher_exact(int n11, int n12, int n21, int n22, double *_left, double *_right, double *two)
63 double p, q, left, right;
67 n1_ = n11 + n12; n_1 = n11 + n21; n = n11 + n12 + n21 + n22; // calculate n1_, n_1 and n
68 max = (n_1 < n1_) ? n_1 : n1_; // max n11, for right tail
70 if (min < 0) min = 0; // min n11, for left tail
71 *two = *_left = *_right = 1.;
72 if (min == max) return 1.; // no need to do test
73 q = hypergeo_acc(n11, n1_, n_1, n, &aux); // the probability of the current table
75 p = hypergeo_acc(min, 0, 0, 0, &aux);
76 for (left = 0., i = min + 1; p < 0.99999999 * q; ++i) // loop until underflow
77 left += p, p = hypergeo_acc(i, 0, 0, 0, &aux);
79 if (p < 1.00000001 * q) left += p;
82 p = hypergeo_acc(max, 0, 0, 0, &aux);
83 for (right = 0., j = max - 1; p < 0.99999999 * q; --j) // loop until underflow
84 right += p, p = hypergeo_acc(j, 0, 0, 0, &aux);
86 if (p < 1.00000001 * q) right += p;
90 if (*two > 1.) *two = 1.;
91 // adjust left and right
92 if (abs(i - n11) < abs(j - n11)) right = 1. - left + q;
93 else left = 1.0 - right + q;
94 *_left = left; *_right = right;
101 int main(int argc, char *argv[])
104 int n11, n12, n21, n22;
105 double left, right, twotail, prob;
107 while (scanf("%s%d%d%d%d", id, &n11, &n12, &n21, &n22) == 5) {
108 prob = kt_fisher_exact(n11, n12, n21, n22, &left, &right, &twotail);
109 printf("%s\t%d\t%d\t%d\t%d\t%.6g\t%.6g\t%.6g\t%.6g\n", id, n11, n12, n21, n22,
110 prob, left, right, twotail);