convolution FFT2D question for comparing results

do not understand how validate the comparison of results between GPU and CPU in the sample “convolutionFFT2D”:

double sum_delta2 = 0;
double sum_ref2   = 0;
double max_delta_ref = 0;

for (int y = 0; y < dataH; y++)
    for (int x = 0; x < dataW; x++)
    {
        double  rCPU = (double)h_ResultCPU[y * dataW + x];
        double  rGPU = (double)h_ResultGPU[y * fftW  + x];
        double delta = (rCPU - rGPU) * (rCPU - rGPU);
        double   ref = rCPU * rCPU + rGPU * rGPU;

        if ((delta / ref) > max_delta_ref)
        {
            max_delta_ref = delta / ref;
        }

        sum_delta2 += delta;
        sum_ref2   += ref;
    }

what kind of test is this?.. xi-square or what?

It appears to be this one: http://en.wikipedia.org/wiki/Student's_t-test#Equal_sample_sizes.2C_equal_variance
though I do not understand the rest.

It is called sum of squared differences (SSD) and is a very common error measure in image processing.