Tools > Merge Data Files > Add New Cases contains various safety mechanisms to avoid discrepancies between data files from causing data integrity problems.
Contradictory Value:Label pairs
For example, if one data file contained a variable called gender with values and labels of 1: Male and 2: Female, while the other contains 1: Female and 2: Male, Q will alert you to the difference and then recode the data in one of the files (based on the option you select in To resolve conflicts) so that the values and labels align. That is, Q assumes that the labels are correct and the values are wrong.
Contradictory Value Labels
For example, if one data file contained a variable called gender with values and labels of 1: Male and 2: Female, while the other contains 1: MALE and 2: WOMAN, Q will alert you to the difference and then recode then create a new variable that synthesizes the two variables (based on the option you select in To resolve conflicts), containing all the unique labels. For example: 1: Male, 2: Female, 3: MALE and 4: WOMEN.
Contradictory Variable Labels
For example, if one data file contained a variable called gender with a Label of Sex, while the other contains a Label of Gender, Q will warn you that Labels differ and then use the appropriate label based on your selection for To resolve conflicts.