Effective interpretation and analysis of records requires the genealogist to know the limitations of the record being used. These limitations generally concentrate on who does not get listed in the record, how the information is given for inclusion in the records, how the records are organized (which impacts how they are searched), etc.
For land records some limitations would be that only land owners are mentioned, some deeds were not recorded promptly, some land ownership happened without a deed being drawn up (eg. inheritance), etc. For obituaries some limitations would be cost of publishing in newspaper (if applicable as not all newspaper required payment), desire of family for others to know, ethnic and social class of deceased, need to search manually if papers are not digitally available, etc.
Knowing the limitations requires the researcher to know something about the record, how it was created, how it was stored, etc. For any record you use, knowing the limitations is crucial to searching efficiently, interpreting responsibly, and using the located record effectively to create an additional research plan.