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This program calculates the chi-square significance values for given
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degrees of freedom and the tail probability (type I error rate) for
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given observed chi-square statistic and degree of freedom.
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Ziheng Yang, October 1993.
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double PointChi2 (double prob, double v);
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#define PointGamma(prob,alpha,beta) PointChi2(prob,2.0*(alpha))/(2.0*(beta))
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#define CDFGamma(x,alpha,beta) IncompleteGamma((beta)*(x),alpha,LnGammaFunction(alpha))
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#define CDFChi2(x,v) CDFGamma(x,(v)/2.0,0.5)
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double PointNormal (double prob);
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double CDFNormal (double x);
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double LnGammaFunction (double alpha);
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double IncompleteGamma (double x, double alpha, double ln_gamma_alpha);
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int main(int argc, char*argv[])
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int i,j, n=20, ndf=200, nprob=8, option=0; /* 0:table, 1:prob */
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double df,chi2, d=1.0/n, prob[]={.005, .025, .1, .5, .90, .95, .99, .999};
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printf ("\nd.f. & Chi^2 value (Ctrl-c to break)? ");
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scanf ("%lf%lf", &df, &chi2);
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if(df<1 || chi2<0) break;
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prob[0]=1-CDFChi2(chi2,df);
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printf ("\nprob = %.9f = %.3e\n", prob[0],prob[0]);
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printf ("\n\nChi-square critical values\n");
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for (i=0; i<ndf; i++) {
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printf ("\n\t\t\t\tSignificance level\n");
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for (j=0; j<nprob; j++) printf ("%9.4f", 1-prob[j]);
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printf ("\n%3d ", i+1);
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for (j=0; j<nprob; j++)
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printf ("%9.4f", PointChi2(prob[j],(double)(i+1)));
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if (i%5==4) printf ("\n");
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printf ("\nENTER for more, (q+ENTER) for quit... ");
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if (getchar()=='q') break;
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double PointNormal (double prob)
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/* returns z so that Prob{x<z}=prob where x ~ N(0,1) and (1e-12)<prob<1-(1e-12)
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returns (-9999) if in error
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Odeh RE & Evans JO (1974) The percentage points of the normal distribution.
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Applied Statistics 22: 96-97 (AS70)
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double a0=-.322232431088, a1=-1, a2=-.342242088547, a3=-.0204231210245;
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double a4=-.453642210148e-4, b0=.0993484626060, b1=.588581570495;
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double b2=.531103462366, b3=.103537752850, b4=.0038560700634;
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double y, z=0, p=prob, p1;
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p1 = (p<0.5 ? p : 1-p);
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if (p1<1e-20) return (-9999);
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y = sqrt (log(1/(p1*p1)));
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z = y + ((((y*a4+a3)*y+a2)*y+a1)*y+a0) / ((((y*b4+b3)*y+b2)*y+b1)*y+b0);
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return (p<0.5 ? -z : z);
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double CDFNormal (double x)
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/* Hill ID (1973) The normal integral. Applied Statistics, 22:424-427.
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adapted by Z. Yang, March 1994. Hill's routine is quite bad, and I
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Adams AG (1969) Algorithm 39. Areas under the normal curve.
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Computer J. 12: 197-198.
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double p, limit=10, t=1.28, y=x*x/2;
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if (x<0) { invers=1; x*=-1; }
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if (x>limit) return (invers?0:1);
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p = .5 - x * ( .398942280444 - .399903438504 * y
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/(y + 5.75885480458 - 29.8213557808
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/(y + 2.62433121679 + 48.6959930692
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/(y + 5.92885724438))));
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p = 0.398942280385 * exp(-y) /
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(x - 3.8052e-8 + 1.00000615302 /
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(x + 3.98064794e-4 + 1.98615381364 /
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(x - 0.151679116635 + 5.29330324926 /
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(x + 4.8385912808 - 15.1508972451 /
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(x + 0.742380924027 + 30.789933034 /
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(x + 3.99019417011))))));
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return invers ? p : 1-p;
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double LnGammaFunction (double alpha)
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/* returns ln(gamma(alpha)) for alpha>0, accurate to 10 decimal places.
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Stirling's formula is used for the central polynomial part of the procedure.
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Pike MC & Hill ID (1966) Algorithm 291: Logarithm of the gamma function.
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Communications of the Association for Computing Machinery, 9:684
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double x=alpha, f=0, z;
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return f + (x-0.5)*log(x) - x + .918938533204673
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+ (((-.000595238095238*z+.000793650793651)*z-.002777777777778)*z
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+.083333333333333)/x;
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double IncompleteGamma (double x, double alpha, double ln_gamma_alpha)
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/* returns the incomplete gamma ratio I(x,alpha) where x is the upper
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limit of the integration and alpha is the shape parameter.
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returns (-1) if in error
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ln_gamma_alpha = ln(Gamma(alpha)), is almost redundant.
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(1) series expansion if (alpha>x || x<=1)
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(2) continued fraction otherwise
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Bhattacharjee GP (1970) The incomplete gamma integral. Applied Statistics,
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double p=alpha, g=ln_gamma_alpha;
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double accurate=1e-8, overflow=1e30;
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double factor, gin=0, rn=0, a=0,b=0,an=0,dif=0, term=0, pn[6];
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if (x==0) return (0);
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if (x<0 || p<=0) return (-1);
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factor=exp(p*log(x)-x-g);
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if (x>1 && x>=p) goto l30;
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/* (1) series expansion */
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term*=x/rn; gin+=term;
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if (term > accurate) goto l20;
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/* (2) continued fraction */
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a=1-p; b=a+x+1; term=0;
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pn[0]=1; pn[1]=x; pn[2]=x+1; pn[3]=x*b;
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a++; b+=2; term++; an=a*term;
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for (i=0; i<2; i++) pn[i+4]=b*pn[i+2]-an*pn[i];
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if (pn[5] == 0) goto l35;
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rn=pn[4]/pn[5]; dif=fabs(gin-rn);
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if (dif>accurate) goto l34;
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if (dif<=accurate*rn) goto l42;
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for (i=0; i<4; i++) pn[i]=pn[i+2];
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if (fabs(pn[4]) < overflow) goto l32;
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for (i=0; i<4; i++) pn[i]/=overflow;
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double PointChi2 (double prob, double v)
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/* returns z so that Prob{x<z}=prob where x is Chi2 distributed with df=v
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returns -1 if in error. 0.000002<prob<0.999998
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Best DJ & Roberts DE (1975) The percentage points of the
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Chi2 distribution. Applied Statistics 24: 385-388. (AS91)
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Converted into C by Ziheng Yang, Oct. 1993.
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double e=.5e-6, aa=.6931471805, p=prob, g;
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double xx, c, ch, a=0,q=0,p1=0,p2=0,t=0,x=0,b=0,s1,s2,s3,s4,s5,s6;
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if (p<.000002 || p>.999998 || v<=0) return (-1);
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g = LnGammaFunction (v/2);
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if (v >= -1.24*log(p)) goto l1;
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ch=pow((p*xx*exp(g+xx*aa)), 1/xx);
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if (ch-e<0) return (ch);
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q=ch; p1=1+ch*(4.67+ch); p2=ch*(6.73+ch*(6.66+ch));
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t=-0.5+(4.67+2*ch)/p1 - (6.73+ch*(13.32+3*ch))/p2;
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ch-=(1-exp(a+g+.5*ch+c*aa)*p2/p1)/t;
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if (fabs(q/ch-1)-.01 <= 0) goto l4;
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p1=0.222222/v; ch=v*pow((x*sqrt(p1)+1-p1), 3.0);
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if (ch>2.2*v+6) ch=-2*(log(1-p)-c*log(.5*ch)+g);
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if ((t=IncompleteGamma (p1, xx, g))<0) {
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printf ("\nerr IncompleteGamma");
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t=p2*exp(xx*aa+g+p1-c*log(ch));
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s1=(210+a*(140+a*(105+a*(84+a*(70+60*a))))) / 420;
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s2=(420+a*(735+a*(966+a*(1141+1278*a))))/2520;
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s3=(210+a*(462+a*(707+932*a)))/2520;
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s4=(252+a*(672+1182*a)+c*(294+a*(889+1740*a)))/5040;
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s5=(84+264*a+c*(175+606*a))/2520;
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s6=(120+c*(346+127*c))/5040;
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ch+=t*(1+0.5*t*s1-b*c*(s1-b*(s2-b*(s3-b*(s4-b*(s5-b*s6))))));
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if (fabs(q/ch-1) > e) goto l4;