In Canadamany and the United Statescountries, the term '''principal investigator''' ('''PI''') refers to the holder of an independent grant and the lead researcher for the grant project, usually in the sciences, such as a laboratory study or a [[clinical trial]]. The phrase is also often used as a synonym for "head of the laboratory" or "research group leader". While the expression is common in the sciences, it is used widely for the person or persons who make final decisions and supervise funding and expenditures on a given research project.<ref>{{Cite web|date=February 2021|title=NERC RESEARCH GRANTS AND FELLOWSHIPS HANDBOOK|url=https://www.ukri.org/wp-content/uploads/2021/07/NERC-20072021-GrantsFellowshipsHandbook2021.pdf|access-date=27 August 2021|website=UK Research and Innovation|archive-date=27 August 2021|archive-url=https://web.archive.org/web/20210827195915/https://www.ukri.org/wp-content/uploads/2021/07/NERC-20072021-GrantsFellowshipsHandbook2021.pdf|url-status=dead}}</ref>
A co-investigator (Co-I) assists the principal investigator in the management and leadership of the research project. There may be a number of co-investigators supporting a PI.<ref>{{cite web |url=https://je-s.rcuk.ac.uk/Handbook/pages/GuidanceonCompletingaStandardG/Co_Investigators.htm |title=Co-Investigators |website=je-s.rcuk.ac.uk}}</ref>▼
▲A co-investigator (Co-I) assists the principal investigator in the management and leadership of the research project. There may be a number of co-investigators supporting a PI.<ref>{{cite web |url=https://je-s.rcuk.ac.uk/Handbook/pages/GuidanceonCompletingaStandardG/Co_Investigators.htm|last=Research Councils |title=Joint Electronic Submissions Handbook: Co-Investigators |websiteaccessdate=je-s.rcuk.ac.uk4 May 2021}}</ref>
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==Federal funding==
In the context of United States [[federal funding]] from agencies such as the [[National Institutes of Health]] (NIH) or the [[National Science Foundation]] (NSF), the PI is the person who takes direct responsibility for completion of a funded project, directing the research and reporting directly to the funding agency.<ref>See, e.g.for NSF Grant Policy Manual 210f 'Definitions: Principal Investigator'example, available: [https://www.nsf.gov/pubs/2002/nsf02151/gpm2.jsp#210 NSF Grant Policy Manual 210f 'Definitions: Principal Investigator'].</ref><ref>[http://www.niaid.nih.gov/ncn/glossary/default5.htm#pi NIAID (NIH) 'Glossary of Funding and Policy Terms and Acronyms']</ref> For small projects (which might involve 1–5 people) the PI is typically the person who conceived of the investigation, but for larger projects the PI may be selected by a team to obtain the best strategic advantage for the project.{{cn|date=November 2023}}
In the context of a clinical trial, a PI may be an academic working with grants from [[NIH]] or other funding agencies, or may be effectively a contractor for a [[pharmaceutical company]] working on testing the safety and efficacy of new medicines.{{cn|date=November 2023}}
There were 20,458 PIs on [[NIH grant#R01|NIH R01 grants]] in US biomedical research in 2000. In 2013, this number grew towas 21,511. At the same time, the success rate for an applicant to receive an R01 grant went down from 32% in 2000 to 17% in 2013.<ref name="Couzin-Frankel">{{Cite journal
