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?