What's the difference between an associate professor and an assistant professor?
What can one of them do that the other can't? and which is a higher level? can any of them supervise a PhD student?
In a typical university in the United States:
An assistant professor is an entry-level faculty member. They are generally on the tenure track (although the term "assistant professor" does not guarantee this) but do not have tenure yet. Typically, within about seven years an assistant professor will either be promoted to associate professor or will leave the university, although the timing can vary a little and it's theoretically possible to remain an assistant professor forever.
An associate professor is one step up from an assistant professor. This promotion is usually the same as getting tenure, but not always. (Some universities, like MIT, frequently have non-tenured associate professors.) The final step for most faculty is a full professorship.
As for what an associate professor can do that an assistant professor can't, that varies even more than the terminology. In many US universities, the only additional power an associate professor has is voting on who gets tenure, but I wouldn't claim this is universally true.
In the Netherlands both assistant and associate professors are frequently tenured (= have a permanent position). Associate professors are expected to develop their own research line, while assistant professors can work on the topics of their bosses (full professors). Neither assistant nor associate professors can formally supervise PhD students: they can only co-supervise. There are some more minor differences: e.g., associate professors can be members of the Ph.D. assessment committee, assistant - not, unless they are co-supervisors of the candidate.
Update 2018: As of last year associate professors at some universities have been granted the right to formally supervise PhD students. Details of the implementation are however left to the universities, and, e.g., Eindhoven decided to grant this right only to senior associate professors.
In Australia, the typical hierarchy is:
In the typical Australian ranking system, there is no "assistant professor" . In this academic ranking system associate professor is a high ranking. I think that both associate professor and professor in Australia would correspond roughly to professor in the United States.
As is noted in the comments, a small number of Australian universities have adopted (or adopted and then reverted back from) the American system. So you may find Assistant Professor is used occasionally. Assistant Professor probably maps onto Australian Level B (Lecturer).
Supervision of a PhD student depends on university regulations. At my university in Australia, there are several requirements in order to be a principal supervisor. In particular, (a) you need to have completed your own PhD or in rare cases be of equivalent standing, (b) have been an associate supervisor of PhD student to completion, or completed a set of training and experiential activities.
In France, the position Assistant Professor is a permanent position. As research activities are also done in labs in addition to the universities, the positions are:
assistant professor (maître de conférences) : permanent teaching position, but can not supervise PhD students alone.
Is typically working towards a habilitation (HDR, habilitation à diriger des recherches), a longer-lasting standalone research project of about 5 years, during which the person co-supervised a few PhD students. After defending the project in front of the jury a person with an HDR can supervise PhD students alone.
professor (professeur des universités) : permanent teaching position, can supervise PhD students alone.
full researcher (chargé or directeur de recherche) : permanent research position without teaching, can supervise PhD students alone if holder of the HDR.
In the Czech Republic, and I suppose that in some other "former Eastern block" countries it's similar, there are 3 types of university permanent positions:
asistent -- usually translated assistant professor
docent -- usually translated associated professor -- you become a docent when you do your habilitation
profesor -- usually translated full professor -- you become a profesor when the president of the Czech Republic promotes you.
There are no other distinguished levels at the universities. However, the Academy of Sciences is completely seperated, and it's different there. Nobody is, however, called a professor there.
In Uruguay, it is not really related to tenure:
The last 2 are more administrative/in charge of lectures than clinical in med-school, though they do perform surgeries with residents.
In Spain you get:
Also bear in mind, that universities in Spain and elsewhere are usually highly politicized environments that conduct regular elections at university, faculty and department levels, so a Professor Researcher might be Dpt. Dr. for some years, then be a Researcher and later be elected as Rector, finally staying as Cathedratic, for instance. the highest staff rotation occurs at the lower levels, while the top levels are the most political. Of course, technical and scientific knowledge plays a role of paramount importance as well. The situation, however, may differ from one university to the other, particularly according to their size and their public or private nature.
Supervision of PhD students is usually performed by levels 3, 4 and 5, with some cases of level 6 and above. Actually, the more the thesis supervised, the more likely it is to climb up the ladder.
Remember as well that, in Spanish, there exists a single word for both the terms "teacher" and "professor", which is "profesor" (with the accent on the last syllable), so denominations may vary; for instance: all High School teachers are generally called "profesor" even if they didn't earn a doctorate degree ("doctorado"). This is so again because of the latin origin of the verb "profesar" which means "the one who declares, or speaks publicly" and is typically use as a synonym to "perform" (some kind of job or occupation) with devotion and commitment.
In France, there is no position equivalent to assistant professor. The position closest to associate professor is a permanent position.
Maître de conférences (MCF, ~ associate professor): Permanent research and teaching position. Can not supervise PhD students alone without an HDR (see next point). Can serve on committees for recruiting new MCFs in the university.
Habilitation à Diriger des Recherches (HDR): A longer-lasting standalone research project of several years, sometimes requiring to have co-supervised a few students. Not a position per se, but allows to supervise PhD students alone and is often necessary to move on to the next position.
Professeur des universités (PR, ~ full professor): Senior permanent research and teaching position. In most cases, requires an HDR (which is still necessary to supervise PhD students alone). Can serve on committees for recruiting new MCFs and PRs.
In national research institutes such as the CNRS, two equivalent positions exist: Chargé de recherche (CR, "Junior Scientist") and Directeur de recherche (DR, "Research Director", "Senior Scientist"). Both are roughly equivalent to maître de conférences and professeur, respectively; one main difference is that there is no teaching load for these positions. An HDR is also necessary in most cases to be promoted directeur de recherche. An MCF can become DR (rather rare as far as I understand?), and a CR can become PR.
For more information see the Wikipedia article Academic ranks in France.
In NIGERIA the academic positions are in the following order of hierarchy:
There is therefore no "assistant professor" in the academic ranking system in Nigeria. However, by comparison associate professor is a higher ranking of about 7-9 years above Assistant Professor. In my opinion, I think both Associate Professor and Professor in Nigeria would roughly equals to Professor in the United States; similar to the situation in Australia. The UK university/academic ranking system is practised in Nigeria.
Supervision of a PhD depends on university regulations. At my university in Nigeria, there are several requirements in order to be a lead supervisor of a PhD, in particular, (i) you need to have completed your own PhD with relevant post qualification experience, (ii) have been a member of supervisory committee(s) of PhD candidate(s) to completion, or completed a set of training activities in related research area.
Promotions from one academic rank to another takes a minimum of three (3) years in addition to the required or relevant research outputs and publications. There are guidelines by each university on the minimum requirements for promotions across the academic hierarchy.
In Germany, faculty belong to a special class of civil servants known as Beamter and the positions they hold are categorized according to the labels they are given in the pay system for the civil servants: W1, W2, and W3, respectively.
W1 positions are always untenured positions that may offer a tenure-track option and correspond to assistant professorships in the US. In some states, after a positive interim evaluation, they are eligible to be the supervisor of record for a thesis defense. Since three years is typically the minimum time needed for a PhD, this basically means a W1 professor can begin supervising students as soon as she is hired. The position is normally term-limited to six years, but this can be extended by the university.
W2 professorships vary; some are term-limited while others may be permanent; they are the “associate” professors of Germany. They are functionally the same in terms of duties and responsibilities in most states, but how the position is reckoned in terms of providing budget support from the university differs. Some universities count W2’s and others don’t, resulting in significant differences in the level of budget support.
W3 positions carry the most responsibility and correspond to the US full professor. They receive the lion’s share of funding and support, and some of the “chairs” or “institutes” they oversee can have dozens or even hundreds of researchers. To some extent they can be closer to CEO’s than faculty, except for the requirement of direct contact hours with students giving lectures and overseeing research. (Typically, they may have teaching loads of 12 to 15 contact hours per week.)
There are also other positions within the system known as außerplanmäßige Professoren that correspond to adjuncts and lecturers, but they do not have lifetime appointments.
In Venezuela, both assistant and associate exist, and both are full professors, but you move from one category to another due to the amount and quality of research you perform, as well as the degrees they hold:
The positions are:
Instructor : Mostly, they are in charge of teaching up to three subjects or three classes, conducting researches and participating in community service projects as tutors. This position is based on tenure (0-2+ years).
Asistente (Assistant): They are in charge of teaching up to three subjects or three classes, conducting researches, participating in community service projects as tutors, tutoring another researchers, being part of an academic group (Student Recruitment, Post-graduate Programs, Community Service Development, Cultural Affairs, etc). Professors get to this position once they hold a master's degree.
Agregado (Aggregate): They are in charge of the same responsibilities as an Assistant Professor, but can be the Head of a Subject/Area and a Lab Coordinator. Professors get to this position either holding a master’s degree or conducting a research that gets reviewed and approved by some experts (professors from higher positions, including the Dean). They can also get to this position by getting a research paper published in a distinguished academic journal. Papers are also reviewed and approved by some experts from the Faculty (professors from higher positions, including the Dean).
Asociado (Associate): They are in charge of the same responsibilities as an Aggregate Professor. Professors get to this position once they get two research papers published in a distinguished academic journal. Papers are also reviewed and approved by some experts from the Faculty (professors from higher positions, including the Dean).
Titular: They are in charge of the same responsibilities as an Aggregate Professor, but they can be Head of a Department. Professors get to this position once they hold a PhD, but they can also get to this position by getting three research papers published in a distinguished academic journal. Papers are also reviewed and approved by some experts from the Faculty (professors from higher positions, including the Dean).
To get to those positions, a professor must have gotten a full contract. According to their contract type, they can be:
Tiempo Convencional: Teaching hours vary from 6, 8, 10 or 12. They do not hold any other academic/administrative responsibility beyond teaching. They are all Instructors.
Medio Tiempo: 12 Teaching hours + 6 hours that can be for research or community service tutoring. They are all Instructors.
Tiempo Completo: 35 working hours weekly (12 for teaching + 23 for the other academic/administrative responsibilities). They must be at least Assistants.
Dedicación Exclusiva: 38 working hours weekly (12 for teaching + 26 for the other academic/administrative responsibilities). They are not allowed to have another job. They must be at least Aggregate.
In India, at regional universities and institutes, Assistant professor is an entry level position in colleges. For this you need to have a master degree. After 4-5 years of experience as an Assistant Professor you may be promoted to Associate Professor. That depends on your knowledge and passion in this field.
If you have a Ph.D. in any subject, then you can directly become an Associate professor.
If you want to become a Professor you need to have at least 3-5 years of experience as an Associate Professor with PhD.
You can do Ph.D. along with your Assistant Professor Career.
The positions are in the following order:
However, at top-notch institutes (like IITs and IISc etc.) or the best universities ( Delhi Univ., Cal. Univ.), a faculty member must hold a Ph.D. degree. In fact, in IITs, at least 2 post-docs from reputed foreign schools are necessary even for an entry level assistant Prof.
In the United Kingdom, the terminology is different from the US:
Senior lecturer (equivalent to associate professor)
Lecturer or clinical lecturer (equivalent to assistant professor)
For more details, see here.
At the University of Wyoming, associate professors have tenure while assistant professors do not. Full professorship is a time thing, not a standing thing here. We have associate professors as department chairs, as doctoral advisors, and they can vote on tenure. Full professors just have a raise and have been there awhile.