In order to understand the strength of each piece of evidence within Italian genealogical documents, one must understand the basics of genealogical evidence analysis and the cultural practices that surrounded the creation of the records.
The best book to begin your studies on genealogical evidence analysis is Elizabeth Shown Mill's Evidence Explained Citing History Sources from Artifacts to Cyberspace. The first chapter covers this subject and the rest of the book shows you how to cite any type of record you may encounter within genealogy. The examples for Italian records are limited but do cover the basics of citing civil records.
Direct evidence is any piece of evidence that answers the research question by itself. Therefore, a civil birth record that states the child was born on 2 March 1856 at 10:00 a.m. in the town of Altomonte is clearly direct evidence of said child's birthdate, place, and time.
Indirect evidence is any piece of evidence which, by itself, does not answer the research question but needs correlation with other evidence. Therefore, a parish marriage record documenting the marriage of Simone Mattei and "the girl Isabella" provides indirect evidence of Isabella's full name. Her surname was not given so additional evidence would need to be gathered in order to determine her full name.
Negative evidence occurs when information should be there but isn't. Because of this, we then infer our genealogical conclusion by the lack of clearly stated evidence. For example, the death record of Carmelo Catanese gives his parent's names as Simone and Maria Abbate. The sections for occupation and residence following Simone's name are crossed through. While no word precedes Simone's name to indicate he was deceased, the fact that they did not insert an occupation and residence infers that he was deceased and provides negative evidence of this conclusion.
The next post on this subject will discuss sources and information, also valuable subjects to understand when working in Italian records.