I am assuming you mean how you would use the Family Bible as a resource. If that is the case, here are some pointers:
- No information should be taken at face value and should be thoroughly researched and verfifed to be accurate.
- Some family bibles are only partially documented. They should be used as clues to further research rather than fact.
- Compare the dates to that of the bible itself. Any dates that proceed the publishing or year of the bible were obviously not recorded at the time of the event. A birth date of 1880 in a Bible with a publication date of 1920 means that the birth wasn’t entered for at least 40 years, so the entry very well could contain errors (see #1).
- If the ink and handwriting is nearly identical, it is very possible that entries were made at one time (or in groups in a 'catch up session') after the event(s) and should be scrutinized as such (see #1).
- Are family bibles primary or secondary sources? This is probably an overlooked question - and the truth is - it depends. Just like any other source, the annotated pages of a bibles contain information that is subject to oversight or error (see #1 and #2). We may also error as well in our ability to fully interpret (or translate) that information into useful data. This will also depend on the original author and their role(s) in notated events. Was your grandmother the author of the notes, and a witness or primary party in documenting her children's and grandchildren's births? Or was it Uncle Bob, who received information in a letter a month later, who eventually recorded the information in his bible based on what he was told? (See #1).
In general, I would consider family bibles to be great research tools that may give you clues into certain details that may have previously been unknown, or confirm the details that you may already know - but should not be considered as "fact" without further research and sources.
I hope that is the direction you were looking for.