Even though the FDA does not have hard rules for human factors, only guidelines (as per the link given by Luisa).The FDA expects manufacturers to engage in human factors for risk management and it is very important. The human factors focus for risk management is the use-related errors. Risk management of use-related errors deals with the identification and description of use scenarios that could result in hazards and the assessing of these risks. To asses these risks it is needed user interface design changes that eliminate or mitigate risks. Then by verifying that the design changes do mitigate risk and that no new hazards are introduced they can validate that the final device can be used safely and effectively. Device manufacturers shall definitely perform human factor evaluations to ensure that the device can be used safely and effectively, and that they can justify their level of human factors effort to the FDA if asked.