public double getCachedValue(ContingencyTable ct)
{
int[] aKey = new int[] { ct.getA(), ct.getB(), ct.getC(), ct.getD() };
if (m_slContingencyTables.ContainsKey(aKey))
{
return(m_slContingencyTables[aKey]);
}
return(double.NaN);
}
/**
* Computes the hypergeometric probablity using the following factorization:
* (a+b)!(a+c)!(b+d)!(c+d)! (a+b)! (a+c)! (c+d)! (b+d)!
* ------------------------ = ------- * ------- * -------- * --------
* a!b!c!d!n! a!b! c! d! n!
* The assumption is that (a+b) is the smallest marginal.
* A better implementation would check for the smallest marginal and factor according to it, but the current implementation seems fast enough.
* */
public static double pr(ContingencyTable ct)
{
double pt = 1;
double iFactorial = 0;
int a = ct.getA(), b = ct.getB(), c = ct.getC(), d = ct.getD();
double iDenominator = a + b + c + d;
double iMinDenominator = b + d;
for (iFactorial = a + 1; iFactorial <= a + b; iFactorial++) // (a+b)!/a!b!
{
pt *= iFactorial / (iFactorial - a);
while ((pt > 1) && (iDenominator > iMinDenominator))
{
pt /= iDenominator;
iDenominator--;
}
}
for (iFactorial = c + 1; iFactorial <= a + c; iFactorial++) // (a+c)!/c!
{
pt *= iFactorial;
while ((pt > 1) && (iDenominator > iMinDenominator))
{
pt /= iDenominator;
iDenominator--;
}
}
for (iFactorial = d + 1; iFactorial <= c + d; iFactorial++) // (c+d)!/d!
{
pt *= iFactorial;
while ((pt > 1) && (iDenominator > iMinDenominator))
{
pt /= iDenominator;
iDenominator--;
}
}
if (pt == 0.0) //underflow
{
return(double.Epsilon);
}
while ((iDenominator > iMinDenominator) && (pt > 0.0))
{
pt /= iDenominator;
if (pt == 0.0) //underflow
{
return(double.Epsilon);
}
iDenominator--;
}
if (pt > 1.0) //numerical error
{
pt = 1.0;
}
return(pt);
}
public void setCachedValue(ContingencyTable ct, double dValue)
{
int[] aKey = new int[] { ct.getA(), ct.getB(), ct.getC(), ct.getD() };
if (!m_slContingencyTables.ContainsKey(aKey))
{
m_slContingencyTables.Add(aKey, dValue);
}
else
{
m_slContingencyTables[aKey] = dValue;
}
}