We define that a probe encodes the base b if b is located in the centre-most position of the probe and is the base to be To validate if the observed PM probe encoding b 1 is indeed the true PM probe of the sample sequence, we compute the likelihood ratio of f obs and f rand , where f obs is probability of observing the hybridization intensity reduction order b 1 b 2 b 3 b 4 given that the PM probe encodes b 1 and f rand is the probability of observing the hybridization intensity reduction order b 1 b 2 b 3 b 4 by chance. Precisely,