Probability of clustering
We calculated the probability of clustering if these three cases were independent events. Age standardised incidence rates from the USA (3) suggest the annual incidence of SSNHL in a 30-year-old (first case) is 12 per 100,000; for a 61-year-old (second case) is 47 per 100,000 and for a 24-year-old (third case) is 12 per 100,000. Discounting the index case, the combined per-person probability of a second and third case occurring in the space of a year, multiplied by the 28 regular staff working in the audiology department at the time, gives an overall probability of:
47/100,000 * 12/100,000 * 28 = 1.6 x 10-6
Therefore, it is highly unlikely that these are independent events.