I recently gave a talk at an internal seminar on planned missingness for a group of developmental psychologists. The idea behind planned missingness is that you can shorten interview time or reduce costs, if you decide as a researcher not to administer all your instruments to everyone in your sample. When you either randomly assign people to receive a particular instrument, or do so by design (i.e. only collect bio-markers in an at-risk group), your missing data will either be Missing Completely At Random (MCAR) or Missing at Random (MAR).