Human factors have been a constant thorn in the side of the designers of military simulations – whereas political-military simulations are often required by their nature to grapple with what are referred to by modellers as "squishy" problems, purely military models often seem to prefer to concentrate on hard numbers. Whilst a warship can be regarded, from the perspective of a model, as a single entity with known parameters (speed, armour, gun power, and the like), land warfare often depends on the actions of small groups or individual soldiers where training, morale, intelligence, and personalities (leadership) come into play. For this reason it is more taxing to model—there are many variables that are difficult to formulate. Commercial wargames, both the tabletop and computer variety, often attempt to take these factors into account: in "", for example, units will generally rout from the field rather than stay to fight to the last man. One valid criticism of some military simulations is these nebulous human factors are often ignored (partly because they are so hard to model accurately, and partly because no commander likes to acknowledge men under his command may disobey him). In recognition of this shortcoming, military analysts have in the past turned to civilian wargames as being more rigorous, or at least more realistic, in their approach to warfare. In the United States, James F. Dunnigan, a prominent student of warfare and founder of the commercial tabletop wargames publisher Simulations Publications Incorporated (SPI, now defunct), was brought into the Pentagon's wargaming circle in 1980 to work with Rand and Science Applications Incorporated (SAI) on the development of a more realistic model. The result, known as "SAS" ("S"trategic "A"nalysis "S"imulation), is still being used.