Definition
The frequency at which a single product or service is completed at the end of a process, i.e., the interval between two finished units coming out one after the other; it is a field in the value stream data box, not inventory days; the official English instructor guide writes it as Drop Off Rate (D/R).
In detail
DOI is most often misread as days of inventory, and the two have nothing in common: it is measured in seconds and captures how often this operation issues a unit. The courseware asks for it to be observed rather than calculated, and gives one criterion — DOI should be shorter than takt time, otherwise the operation cannot keep up with customer demand. Its relationship to process time is the key to spotting parallel work: with no parallel work and only one value object in the process they are equal; when several people do the same thing at once, process time stays the same while DOI gets shorter.
Example in practice
A cutting operation takes 240 seconds per piece, but five machines run in parallel and each sheet yields four pieces, so a unit is observed leaving every 12 seconds and the data box records DOI equal to 12 seconds. Takt is 18 seconds, so this operation keeps up.