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It was thought at first that the 4+5 rectangle would be better, especially allowing for the greater time taken to make a 1+2 rectangle (R1 pp 35, 36). A 2+5 and a 4+5 rectangle could both be made, so as to obtain independent evidence for Χ5 (R1, p 48). In this case one would naturally have a 1+2 rectangle also, but it was decided that the extra trouble was not compensated for by the slightly increased power. For references to 3+4x rectangles see R3 p 7.
For a 'pseudo 2+5 rectangle' see R3 pp 81, 86. The method may be suitable for the case of Χ2 and P5 limitations, but limited statistics tended to show than an ordinary 2+5 rectangle would be better. Another idea that was put forward was a Δ2 rectangle or a 2-impulse bigram rectangle. This also was not encouraged by the statistics. (R3 pp 44, 52, 58.)
(b) Rectangles with Χ2 limitation.
As early as R1 p 59 it was thought that the Χ2 limitation might have a characteristic effect on rectangles. On p 62, R1 there was a reference to a suggestion for a 'repeats' rectangle in which .., .x, x. and xx would be treated separately. This makes sense for Χ2 limitation but not for other limitations (see R2 p 96).
The difference x - x* between the score of a converged rectangle and the score on the correct wheels tends to be greater when the limitation is Χ2.
Methods for diagnosing Χ2 limitation from a converged rectangle were suggested and discussed in R3 pp 59, 101, 119, 122, 123, 126, 128, R4 pp 31, 35, 38. More to the point is a note in R4 p 71 (see also R4 p 80, R5 p 38). It is pointed out here that a 4 l.c. provides evidence about the limitation and that this is so even if a complete Χ2 is assumed, because it will tend to be wrong at Χ2 dots rather than crosses.
(c) Wheel-sliding.
In the very early unsuccessful attempts on Jellyfish the following method was used. Several rectangles of messages on the same month were accurately converged. Then the relative positions of say Χ2 were looked for by sliding the pippages from one rectangle against those of another.
The crudest method of wheel-sliding is to express the wheels in dots, crosses and doubts and to insist on an excess of say 6 or more agreements than disagreements, or vice versa. (Remember that it would not usually be known
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