TABLE 2.

Probability that the MIC will increase every week in exposed individuals

Initial MIC (mg/liter)	Probability of a final MIC (mg/liter) of:
	0.0625	0.125	0.25	0.5	1	2	4	8	16	32
0.0625	1 − (…)b	pa	p/4	p/16	p/64	p/256	0	0	0	0
0.125		1 − (…)	p/2	p/8	p/32	p/128	0	0	0	0
0.25			1 − (…)	p/4	p/16	p/64	p/256	0	0	0
0.5				1 − (…)	p/8	p/32	p/128	0	0	0
1					1 − (…)	p/16	p/64	p/256	0	0
2						1 − (…)	p/32	p/128	0	0
4							1 − (…)	p/64	p/256	0
8								1 − p/128	p/128	0
16									1 − p/256	p/256
32										1

^ap is the probability that strains colonizing an individual will mutate from a susceptible level to the lowest nonsusceptible level: p = P(0.063 → 0.125). Because the MIC is measured on a logarithmic scale, we supposed that P(0.125 → 0.25) = p/2, P(0.25 → 0.5) = p/4, and so on. p was fixed to fit historical data on the emergence of S. pneumoniae resistance between the introduction of penicillin in the 1950s and 1993 (p = 2 × 10⁻⁵).

^b(…) defines, for each row, the sum of all increasing MIC probabilities. Thus, the sum of all probabilities on a given line of the table is equal to 1.