In one of the lessons in Expert level 2 you mention that you advise against using socialsecurity nr as a primary keyfield. Although I understand the advantages of an autonumber, I don't see the disadvantages of using a key such as social securtiy number which is also uniquely identifying an individual (apart from privacy reasons, or database space (social security number is larger than autonumber).  Related to this I have another question: suppose you backup your files by exporting them to excel.  In case you loose your table and want to re-import it back from Excel, I'm assuming the autonumbers will be different.  If then that key is used as a foreign key in another table, it will point to the wrong record.  Does that mean backing up in Excel is a bad idea, or is this an argument pro using more 'meaningful' keys?