Any single entry in the IGI is Undetermined Information from a Derivative Source, following this reasoning (based on the Updated Evidence Analysis Process Map):
- The source ("Container") is Derivative -- it's an alternative presentation of the original that isn't a faithful image. To be Original, it must not be based on prior records.
- The information ("Contents") is Undetermined, until I understand the original source from which the index/transcript entry of interest is derived. As some elements of the IGI were based on PRs or BTs produced by an
eye-witness -- the incumbent of the parish who was present at the
event, even if he didn't record it for some time afterwards -- those
elements contain Primary Information (and perhaps Secondary Information as well, such as the age at death, or the names of parents in a marriage record). However, some elements were based on family history
submissions by other genealogists, usually without any supporting
source information, so those elements have to be classified as
Undetermined or (at best) Secondary, as the information was not
produced by an eyewitness to the event being reported.
The evidence can't be classified without knowing what assertion (research question) it relates to.
If the question is "What was the date of X's baptism" and the entry under consideration is a baptism entry for X in a PR, the evidence is Direct. If the question is "What year was Y born" and the entry under consideration is a burial record for Y with age recorded in a PR, the evidence is still Direct, because it addresses the question directly, even if through a calculation. (However, the information in this case is probably Secondary, given the chances that the person providing the age was present at the birth -- an exception will be the burial of an infant).
If the question is "Who is the mother of Z" and the entry under consideration is an early baptism record for Z that only records the father's name, then it's Indirect evidence because the name of the father has to be combined with other records (perhaps a marriage record) to arrive at a possible name for the mother.
And if the question is "Were Family F non-conformists?" and there are no baptism or burial records for them in the Church of England Parish where they could have been expected to worship, that's Negative Evidence supporting them being non-conformists.
All of this is still not sufficient to tell you how reliable a piece of evidence is, but the necessary process of classifying the evidence, information and source makes you think about how reliable it might be and where the possible sources of error lie.
The book Mastering Genealogical Proof by Thomas W. Jones, although based on American examples, is an excellent study guide to this subject.