You would not restructure the tables, but control what the user does via the forms you set up. For example, if you are providing the list of tests to chose from via a combo box, alter the row source in the combo box based on what has already been entered (you would need a query to do this). Alternatively, you could identify samples perhaps by type and add the tests when the sample is created and then use a little code to prevent the addition of new records. It just depends on how you want to set up your forms.