274
T. BERGAN
example N2B considers the reactions - and + as belonging to one and the
same category, and the remaining reaction strength codes to the other
category so that no difference is recorded for any reactions ranging from
+ L to CL. The number of different patterns in the alternative N2A is 673
and with the alternative N2B there are 481 different patterns.
The most important patterns ensuing with the discrimination alternatives with codes N8 are indicated in Table XXI and with the discrimination
alternative N2A in Table XXII. In the body of the tables each reaction
level as defined in Table XX is indicated by a numerical code. It is seen
that several phage patterns are represented by strains from more than one
TABLE XXI
Consequences of recognising various phage reaction differences as
significant when phage patterns of 3178 strains of S. sonnei are
compared at different levels of discrimination between patterns
Discrimination alternative A : N8" =all degrees of phage reaction differences are
accepted as discriminating between patterns. The eight levels of discrimination
obtained with N8 are
N 8 : l -
2 +
3 + L
4 + +
5 + + +
6 SCL
7 OL
8 CL
With N8, the 3178 cultures are divided into 2156 patterns.
Discrimination alternative B: N2A = only strong reaction differences are accepted
as discriminating between patterns. The strong reactions (more than 80 plaques,
i.e. + + +, SCL, OL, and CL) are combined into one level of type reaction
discrimination, the other patterns are referred to the other level :
N2A: 1 -, +, +", + +
2 + + +, SCL, OL, CL
With N2A, the 3178 cultures are divided into 673 patterns.
Discrimination alternative C: N?B=all reactions of more than 40 plaques are
combined at the one level of discrimination (positive reaction) and the remainder
are combined at the other level of discrimination (negative reaction)
N 2 B : l -, +
2 +L, + f , + + +, SCL, OL, CL
With N2B, the 3178 cultures arC divided into 481 patterns.
a Alternative labelled by number of possible choices.