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I don't have hard numbers (if you know a good source, please do share), but it is quite apparent that there are many more Ph.D. student positions advertised than PostDoc positions, at least in my field (engineering/CS, Europe). My feeling would be that many labs I know have a ratio of maybe 5-10 Ph.D. students to 1 PostDoc. Why is that the case?

Specifically, why do professors or funding organizations prefer hiring a Ph.D. student to hiring a PostDoc? I know that the latter are a little bit more expensive in terms of salary*, but they will generally also be much more productive. (I'm just comparing myself now at the end of my Ph.D. with myself at the beginning of it.) If I were a professor now, had enough money at hand and could choose freely how to spend it, I'd aim for a much lower ratio, maybe 2 Ph.D. students to 1 PostDoc. Why don't people do that? Are there funding constraints (if yes, for which reason)? Aren't there enough qualified applicants (I would doubt this)?

In the larger scheme of things, hiring more senior researchers and less Ph.D. students would also contribute to solving the much lamented (at least in Germany) problem that there is a lack of viable scientific career paths short of becoming a professor.

One final comment: in an earlier question of mine, someone replied that universities have a teaching obligation, thus effectively obliging them to hire Ph.D. students instead of PostDocs. However, in my country (Germany) and field, Ph.D. students are not really seen as students; they are essentially fully paid staff researchers. (They also do not visit any lecture courses.) I do not believe that supervising a Ph.D. student is seen as "fulfilling the university's teaching obligations", although I might be wrong about that.

EDIT: Since the question of ethics and "do people actually want to stay in academia" came up a few times, here are a few articles about the detrimental effects of job scarcity at the postdoc level:

*This will likely vary by region. In another answer, someone claimed that PostDocs are twice as expensive as Ph.D. students. In my institution, however, the pay gap is much lower. Maybe around one fifth or fourth?

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    Assume time as a PhD is 6 years, as a post-doc 3 years. One would expect a 1:3 ratio of post-doc to student. But wait, this presumes all PhDs go on to a post-doc. If only half do, then you are at a 1:6 ratio. Perhaps step back and ask why any given professor has more than one PhD student...
    – Jon Custer
    Aug 16, 2021 at 13:23
  • 2
    You might be interested: Why do departments fund PhD students instead of postdocs?
    – Allure
    Aug 16, 2021 at 20:04
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    Many Ph.D. graduates go on to non-academic careers, for which no postdocs are required.
    – GEdgar
    Aug 16, 2021 at 20:05
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    I've been in places where post-docs outnumbered PhD students 5-to-1, the reason being that the time to be a PhD student is limited but the time to be a postdoc is not.
    – gerrit
    Aug 17, 2021 at 10:00
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    Somewhat related: academia.stackexchange.com/questions/107699/…
    – DJack
    Aug 17, 2021 at 23:01

5 Answers 5

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A few reasons from a European perspective:

  • Graduated PhD students are a success metric: Institutions and departments might be evaluated based on their number of graduated PhD students. Likewise, when a professor goes up for a tenure evaluation or applies for a job somewhere else, they typically need to show a track record of successful supervision of PhD students. Successful post-doc supervision doesn't nearly count as much.
  • Better success chances for post-docs: Successful supervision of PhD students is also a success metrics when post-docs apply for faculty jobs. A research group with a rather high postdoc-to-PhD ratio will give less opportunity for each post-doc to gain the necessary supervision experiences.
  • Ethical reasons: A situation in which every PhD candidate can get a post-doc, but most post-docs have no chance for a professorship might be considered ethically dubious. A post-doc is a temporary position with the goal of strengthening the post-doc's research profile towards a successful application for faculty jobs. Since there are much more people interested in a PhD than there are available professor positions, there has to be a point where "the funnel is narrowed", as to avoid that too many hopeless candidates are stuck in the post-doc stage without any realistic job prospects.
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  • Can I ask what does "A research group with a rather high postdoc-to-PhD ratio will give less opportunity for each post-doc to gain the necessary supervision experiences." mean? What "supervision" here then? Thanks a heap Aug 16, 2021 at 12:54
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    @NoviceMindset At least in my field, it's very common that post-docs co-supervise PhD students. I would go as far as saying that a complete lack of co-supervision opportunities means to set up the post-doc for failure, because it will be hard to build a comparable research track record to other post-docs who have those opportunities. Aug 16, 2021 at 13:11
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    @lighthousekeeper It makes sense now, thanks a heap! Aug 16, 2021 at 13:19
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    Concerning the last point: well, if there were more post-doc positions and less PhD positions, then being a permanent post-doc (i.e., a permanent staff researcher) might become a viable career option - which, sadly, it isn't really right now. A system like the current one where you essentially must become a professor to be a permanent researcher (with a few exceptions) is not the only viable option, or is it?
    – Eike P.
    Aug 18, 2021 at 13:45
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    @jhin It's often internal documents, for example, my own tenure evaluation criteria and a criteria catalog for internal promotions for associate and full professor. I have also seen the number of graduated PhD students been brought up in hiring committee discussions. Aug 18, 2021 at 15:59
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Graduate students are being educated for a broad range of careers, mostly outside academia. There is, after all, a large demand for PhDs in many fields of industry and in government. Universities heed this outside demand by educating many of them.

Postdocs are being educated primarily for academic careers. There is a much smaller demand for them, and so universities hire fewer of them because they would otherwise end up with lots of well-trained postdocs who cannot find jobs for which they have been educated.

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – cag51
    Aug 19, 2021 at 1:21
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It's not an academic reason, it's because the labor supply is saturated pushing up qualifications people are chasing.

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    This is the best answer
    – Tony Ennis
    Aug 18, 2021 at 18:02
  • I don't know, is there really any evidence that people are pursuing PhDs specifically to improve their non-academic job qualifications? It's a really, really narrow slice of jobs that a PhD actually advantages you for, so this argument depends on a lot of PhD-seekers being quite confused. In my experience, PhD students almost all intend to be academics initially. Aug 18, 2021 at 19:36
  • I can speak anecdotally, in ML the FAANG companies basically want to hire PhD graduates even though what they are doing doesn't require it, there's just such a massive number of people graduating in the field at this point that they can set arbitrarily high expectations. The people in my research group are 3/4 international students who hope to get a sponsorship into the US after graduation from these companies. None of them are planning on being professors. Aug 18, 2021 at 19:47
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    @KevinArlin It very much depends on the field. For some STEM positions having a PhD is actually the norm. Whether incoming students are aware of this is another question.
    – Buck Thorn
    Aug 19, 2021 at 8:00
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Hierarchy is everywhere, whether you like it or not:

  1. there are more secondary school graduates than college graduates;
  2. there are more Masters graduates than PhD graduates;
  3. there are more Post-docs than Professors.
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    My question was not about personal preferences, but about understanding the incentives that lead to the current system. More to the point: I totally understand that Nprof > Npostdoc > Nphd > Nmaster will always be true, but this says nothing about the magnitude of the size differences between the different Ns. (As I wrote in the question, my personal best guess would have been around 2-3:1, whereas in practice it often seems to be 5-10:1.)
    – Eike P.
    Aug 18, 2021 at 13:57
  • @jhin you're way overthinking this. Not every person wants to go to the next level; it's as simple as that. (And even some that want to step up see that their job prospects would be limited, of they really need to Get A Job, etc.)
    – RonJohn
    Aug 18, 2021 at 14:22
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    Unfortunately, things do not appear to be as simple as that, considering that there are evidently many people who would like to stay in academia but are forced to drop out. See, e.g., any of these articles.
    – Eike P.
    Aug 18, 2021 at 14:35
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    I am removing some unkind comments; let us remember our code of conduct. That said: I do encourage you to edit your answer to add more detail; in its current form, I think one might be able (correctly) flag it as "not an answer" since you are only restating the premise of the question (that this hierarchy exists). Even just editing your comment above into the text of the answer would make it more answer-like.
    – cag51
    Aug 19, 2021 at 1:16
  • In my alma mater, there are less post-docs than tenured researchers.
    – einpoklum
    Feb 12, 2022 at 21:57
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I would argue that the role of a postdoc is often different from that of a graduate student. I would categorize many such reasons as "managerial".

For instance:

  • Post-docs may be expected to serve as a "back-up" or secondary advisor (sometimes in addition to employing full-time lab managers), particularly in large groups, where some post-docs may take on significant supervisory duties. Having too too many post-docs may result in conflict (too many cooks do spoil the broth).

  • Some professors I have known have displayed distinct signs of "ageism", that is, a preference for younger applicants (purposely or not, this is often encoded in requirements that PhDs be granted only a few years before applying for the postdoc). The experience that seems so admirable can also lead to friction and confrontation under the wrong circumstances.

  • Incoming students are malleable and often more patient than more experienced individuals. They are more willing to follow orders blindly and take on risky projects, in part because they are blank slates, also because they have more time to find and develop a successful project. Ph.D. projects are often supposed to be very high risk (at least historically, although this may be declining). Part of the strategy of discovery is to try something so wild nobody else has bothered to test it before.

  • Postdocs may be highly specialized to either complement or enhance (through teaching) the skills of graduate students. They may be hired for a specific role (rather say than to chase whatever new lead strikes the advisors fancy). There is a natural flow of information here consistent with the educational role of the institution.

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