# How is the inter and the intra distance of a PUF calculated?

I was wondering, if I want to calculate the intra distance of a PUF, and I have already measured $n$ times a response to a challenge. Do I choose one reference value and compare the hamming distance of it to all other values $n-1$ comparisons? Or do I compare each response to each other $(n \times(n-1))/2$ comparisons?

The same question for the inter distance. Furthermore would be interesting how great should be $n$ in both cases to get meaningful values? Can this somhow be statistically "proven" by some kind of tests?

Both metrics and various statistics based on them are, in principle, characterising the performance of the whole PUF construction. For example, the maximum intra-distance would give an idea of the worst case ('least reliable') response that could be expected. As a result, $$n$$ should be equal to the maximum number of challenge-response pairs (CRPs) offered by the PUF.