What is the difference between Human Factors and Human Performance Improvement (HPI)?

There are several places you can look online to define Human Factors.  Wikipedia (https://en.wikipedia.org/wiki/Human_factors_and_ergonomics) states:

“Ergonomics (or human factors) is the scientific discipline concerned with the understanding of interactions among humans and other elements of a system, and the profession that applies theory, principles, data and methods to design to optimize human well-being and overall system performance.”

The Department of Energy (DOE) describes Human Performance Improvement (HPI) as addressing more of the mental part of interacting with a process.  To keep it simple, traditionally Human Factors focused on the ergonomics side of the interaction of humans with processes.  For example: Is the cockpit of an airplane designed such that the pilot can see and reach all of the dials, knobs, handles, gauges, etc., necessary to operator the airplane?  Whereas, HPI started to study the mental interaction with the process.  For example: What factors (latent organizational weaknesses) or conditions (error precursors) exist that are known to exacerbate human fallibility that results in the pilot making errors by operating the process incorrectly (turning the wrong knob or flipping the wrong switch)?

HPI also studies tools and defenses that can be provided to reduce the error, or more importantly reducing the consequences of errors. While Human Factors has always taken this into account to some extent (failure modes and effects analysis), this is the primary focus of HPI when the outcome of an error could result in 1) Injury, 2) Mission Interruption or 3) Damage (i.e. facilities, equipment, environment).

Bottom line is they are complementary to one another.