Can somebody please clear this up for me. My understanding is that the complement of credibility is just (1-Z) and that the ancilliary statistic is the independent information. However, I have noticed a few times the notes/exam solutions do not seem to stick to this, for example, Q9 (i) in April 2014 ASET states "The external information is known as the complement of credibility"
I am afraid that the problem here is that term complement of credibility isn't always uniquely defined. Sometimes it means (1-Z) see the Core Reading on page 3 of Unit 18 and sometimes it means the independent information (or the ancillary statistic) see the Core Reading in Section 4 of Unit 18 which starts on page 32. This demonstrates that it is always important that you clearly define any terms you use in an exam question.
I wouldn't get too bogged in this detail. In general the term "ancillary statistic" and "complement of credibility" can be used interchangeably, to mean "1-Z". To me, it feels less natural for it to mean "the external information", but people would understand what you mean.
