The importance of including subject identifiers cannot be overstated. In some cases, files might contain no identifiers because investigators thought a study would generate only one file. In other studies, records might be identified inconsistently. For example, subjects' names might be used with the spelling or capitalization varying from file to file. In still other studies, some files might use subjects' names as identifiers while others might use sample or recruitment id numbers.
Inadequate identifiers make it difficult to merge files for analysis.
It is highly probable that errors will result due to mismatching. Regardless of the cause or the form of the mismatching, the result will be to compromise weeks, months, or even years of otherwise solid work.