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1. To calculate credibility premium, you first have a loss random variable, say X. X has already encoded the time span you mentioned. For example, X is the number of claims of a policyholder IN ONE YEAR. Please read more examples in your text.
2. First, partial credibility factor is not 0/1082, it should be sqrt(0/1082), although the figures are the same. Back to the question, you are right if you base credibility factor on the number of CLAIMS. Then for full credibility, you charge the policyholder the manual premium (expectation). This makes sense in the way that the policyholder has never told you his/her loss experience (or you never know since he/she does not CLAIM). However, if you calculate credibility based on number of year, say, in the i-th year the claim is X_i, and if the policyholder qualifies for some partial credibility factor Z say after 5 years, then the new premium should be Z*X_bar + (1-Z)*mu = (1-Z)*mu if X_bar is 0. Then there is a discount of Z, coinciding with your intuitions. Note that credibility factor Z would be different if you calculate using different random variables (e.g. number of losses, number of claims, number of policy years, amount of losses, amount of claim payments). It is possible that a policyholder qualifies for full credibility under a certain method but not under another.
By the way, credibility theory (expecially classical) is not very rigorous in mathematics. It does employs maths extensively, but it is rather a practical rule-of-thumb technique rather than a logically-sound theory. Please refer to Page 557 of Loss Model 3rd edition.
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