Principal Investigator
A principal investigator (PI) is the lead researcher for a particular well-defined project, usually in the sciences, such as a laboratory study or a clinical trial.
大意：
壹位“principal investigator（PI）”是某個特定的明確定義項目的首席研究員，通常是指科學領域的，如實驗室研究或臨床試驗等。
Program Director
In service industries, such as education, a program manager or program director researches, plans, develops and implements one or more of the firm's professional services.
大意：
壹名項目經理或項目主管（“Program Director”）通常要研究、計劃、制訂和執行壹個或多個公司的專業服務。通常是指處於服務性行業的，例如教育。
由上述解釋得出，二者最本質區別在於工作領域。
Principal Investigator 通常是指科學領域的；Program Director 通常是指處於服務性行業的；前者常被譯為：首席研究員，後者常被譯為：計劃負責人或項目總監等。