General Report on Tunny
25E Page 169
If such methods leave only a few doubts, wheel characteristics may solve them.
In marginal cases the difficulty is that the highest Χ2dot scores and the lowestΧ2 cross scores may be confused, so that the evidence appears to be conflicting.
The same methods can be used when making chi 2 certain but moderately scoring characters can be tricky because the decibanage for a character depends on whether it is taken as a Χ2 cross or a Χ2 dot. A more precise formulation is given in R4 p57 eqn. Whilst the wheel-breaker is cerebrating, all available runs should be done, for if each supposed Χ2 cross scores 40 decibans more than any supposed Χ2 dot, even when the latter is decibanned as though it were a Χ2 cross, the wheel is certain. For decibanning R3 p 42, R4 pp 57, 104, R5 p 65.
(d) The four-letter count.
As in setting , (23E(h)) a 4-letter count for ΔD1,ΔD2against Χ2 crosses, provides some evidence for the sort of ΔP to be expected. On the whole text is no bulge of x x over .., or vice versa (22H(f)), the bulge against Χ2dots being equal and opposite to that against Χ2 crosses; but at an early stage in wheel-breaking, so many of the ΔΧ 2 characters against Χ2 dots may be wrong that even on the whole text a significant bulge will appear. This will not occur with other limitations and provides additional evidence that the limitation is Χ2.
(e) 
.
From a Χ2 limitation message, it is sometimes possible to break wheels without a rectangle. This depends on ΔΧ2 + Χ2, usually written Χ2 (22A(b), 22D(g)), which has of course a definite value at each position of the chi 2 wheel.
Proportional bulge of (ΔZ2 + = .) = βΠx
where Πx is the P.B. of ΔP2 = x (22H9)
So that if Πx is great enough may be found from the short wheel- breaking run + ΔZ2 = x, the condition for significance being as usual 
> 5.7 (R1 p11, R4 pp 70, 92, R5 p 9.)
This run is made systematically on A-tapes (33A(c)) of links likely to use Χ2 limitation, both on the whole text; and also, in order to detect slides, on thirds. Corruption 9's spuriously enhance the score, so that NOT 99 must be used.
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