Table 2. Observed and expected cluster units of pneumococcal clones in three cohorts of trainees, Israel, 2007.
| Distribution of clonal groups by number of isolates per group | Presence of cluster unitsa in any of the 3 cohorts | Number of cluster units in the current study | ||||
| Number of isolates per group | Observed number of groups | Minimal number needed for cluster unit | P value for being a true cluster unit for each clonal group | Expectedb | Observed | P value of the observed number of cluster units to be beyond simple chance |
| 2 | 14 | 2 | .33 | 4.67 | 8 | .058 |
| 3 | 8 | 2 | .33 | 2.67 | 8 | <.001 |
| 4 | 5 | 3 | .11 | 0.56 | 3 | .012 |
| 5 | 1 | 3 | .09 | 0.09 | 1 | .09c |
| 7 | 3 | 4 | .021 | 0.06 | 3 | <.001 |
| 9 | 1 | 5 | .005 | 0.005 | 1 | .005 |
| Overall | 32 | 8.06 | 24 | <.001 | ||
a
A cluster unit was defined by the presence of >50% of identified clones in one of the 3 cohorts.
b
Expected 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.
c
For 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.