Final answer:
Dr. Howarth is most likely a specialist in psychometrics, which focuses on the measurement and assessment of psychological constructs.
Step-by-step explanation:
The researcher in question is most likely a specialist in psychometrics.
Psychometrics is the branch of psychology that focuses on the measurement and assessment of psychological constructs, such as personality traits, attitudes, and abilities. It involves the design, administration, and interpretation of tests and other measurement instruments to gather data about individuals' psychological characteristics.
In this case, Dr. Howarth's conclusion that the personality test is invalid suggests that he has expertise in evaluating the validity and reliability of psychological tests, which is a key aspect of psychometrics.