In the application we consider here there are two wards and we define two subtypes (TW and non-TW), so both w and j can take values 1 or 2. We also define N w to be the total number of patient episodes on ward w over the study period for which there was at least one MRSA positive swab and M w to be the total number of new MRSA acquisitions on ward w over the study period, i.e. M w~P t P j A wjt . Method 1: Reconstruction of the epidemic tree. The treereconstruction approach calculates the probability that each observed new MRSA acquisition was acquired from each of the other MRSA positive patients in one of the two ICUs. In this approach we condition on the probabilities q ijt and assume that all new acquisitions were acquired from a known patient source. A scaling factor, s, specifies the reduction in the daily risk of transmission from an MRSA positive patient in one ward to an MRSA negative patient in a different ward. We explore the sensitivity of the results to both the q ijt and the s values.