Observed and expected cluster units of pneumococcal clones in three cohorts of trainees, Israel, 2007.

aA cluster unit was defined by the presence of >50% of identified clones in one of the 3 cohorts.bExpected number of cluster units in the current study for each cluster unit size was derived by multiplying the observed number of groups with the same clone and the same size by the P-value for being a true cluster unit.cFor clonal group of 5, if minimal number needed for cluster unit is defined as ≥4 (as was the observed cluster unit) then P value is .037.