From an (unconfirmed) study attributed to the US army Corps of Engineers :
« every time you double your workforce, you decrease your productivity by 20% »
Sayz (in the video below) Essential Craftsman, who also introduced me to the concept of « Shadetree mechanic ».
Study is most probably: U.S. Army Corps of Engineers – Modification Evaluation Impact Guide COE EP 415-1-3 (latest edition: July 1, 1979). Pdf of chapter 4 found online here. According to this 2013 study (pp 11-12), « the 1979 U.S. Army Corps of Engineers “Modification Impact Analysis Guide” (…) is still in use even though it has been formally withdrawn from publication ».