## Captain Walter Fried's Fish Notes Annex to #F 71, Screed on Delta D Counts and Colossus runs## Page 12 |
Tony Sale's Codes and Ciphers |

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- Appendix p2

- = O(B, Z & nA)

The odds on the highest score of a run (3+4)x/1x2x where the wrong X1

might be greater than 1/ 29x26.

- (i) The score for B on (3+4)x may be so enormous that it shews up

even on the random sample of the text given by the wrong

X12 setting. This can be tested by counting (3+4)x.

(ii) A may be wrong, but a good slide of correct settings. This

is a more troublesome possibility.

- This is the ratio of chances of getting the observed scores with

- We shall generally be considering cases where Z contains a pretty good

Let us then also consider It unlikely that P(Z & nB, nA)> P(Z & nB, A).

- On the other hand we may get P(Z & nB, nA)

- These considerations shew that e and m are small compared with 1,

- O(A & B, Z) approx =P(A). O(B,Z & A) = po.

If e is not negligible, it is because X1 or X2 is only a good slide.

- if m is not negligible it is (probaby) because of slides in X3 and X4

are strong rivals on tte second run m is not negligible and po is an

understatement.