Double_t distributionEfficiencyFromOccurence( Double_t *x, Double_t *par){

	// Compute the efficiency for a counting experiment
	// par[0] = N is the number of trials, integer expected
	// par[1] = K is the number of success, integer expected
	// x = p is the success probability
	//
	// Formula is taken from M.Patterno
	
	Double_t p = x[0];
	Double_t N = par[0];
	Double_t K = par[1];
	
	//cout << "x=" << p << " N=" << N << " K=" << K << endl;
	// compute factor (N+1)!/K!/(N-K)! = (N+1)*(N+1-1)*...*(N+1-K)/K/.../1 
	// ratio of K+1 terms over K terms
	// compute the ratio term after term to avoid large factorials
	Double_t factor = N+1;
	for( Int_t i=1; i<=K; i++) {
		factor *= (N-K+i)/i;
	}
	
	//cout << "factors=" << factor << " " << TMath::Power( p, K) * TMath::Power( 1.-p, N-K) << endl;
	
	return factor * TMath::Power( p, K) * TMath::Power( 1.-p, N-K);
}