Table 2.

Means and opportunities for selection calculated for stationary phase densities of clones free of mutations (F) and contaminated with mutations (M)

Pair compared	Sample sizes		Mean stationary densities, OD600		df†	t′(B−A)	Opportunity for selection, ×100		df†	t′(B−A)
A* vs. B*	A	B	A	B			A	B
F30/24h vs. M30/24h	127	229	1.537	1.545	204.9	1.119	0.198	0.227	291.3	−0.475
F38/24h vs. M38/24h	127	228	1.179	1.081	350.6	−4.907**	0.773	7.184	246.5	3.621***
F38/48h vs. M38/48h	127	222	1.150	1.077	327.2	−4.420**	0.394	5.399	226.7	3.363***
F30/24h vs. F38/24h	127	127	1.537	1.179	219.4	−33.04**	0.198	0.773	131.1	2.075
F30/24h vs. F38/48h	127	127	1.537	1.150	250.3	−43.65**	0.198	0.394	149.4	1.463
M30/24h vs. M38/24h	229	228	1.545	1.081	255.7	−25.37**	0.227	7.184	227.3	3.975**
M30/24h vs. M38/48h	229	222	1.545	1.077	258.2	−29.65**	0.227	5.399	221.4	3.486***

^** and *** indicate probability of the type I error lower than 0.001 and 0.01, respectively. Critical values of t′ were calculated for seven planned comparisons. 

^*A and B serve as variables that should be substituted within each row for appropriate F or M, assayed at 30°C or 38°C, after 24 or 48 h. Note that all measurements at 30°C were done only after 24 h, whereas those at 38°C were measured also after 48 h due to slow growth at this temperature. 

^† Degrees of freedom are not natural numbers because they were calculated according to Welsh's procedure to compensate for unequal sizes and variances of the compared samples (19).