In a paper published in the Journal of Ecology in 1993, Jonathan Silvertown, Miguel Franco, Irene Pisanty, and Ana Mendoza analysed elasticities of matrix projection models to quantify the contribution of different life cycle components to population increase rates of 45 herbs and 21 woody species. They found that herbs differed significantly from woody plants in most of these components. For example, seedling recruitment was more important in herbs than in woody plants while the opposite was true for stasis. Twnety-three years after the paper was published, I spoke to Jonathan Silvertown about his motivation to do the work presented in this paper, his memories of this study, and what we have learnt since about this topic.
Citation: Silvertown, J., Franco, M., Pisanty, I., & Mendoza, A. (1993). Comparative plant demography–relative importance of life-cycle components to the finite rate of increase in woody and herbaceous perennials. Journal of Ecology, 465-476.
Date of interview: 22 November 2016
Hari Sridhar: What was your motivation to do the work that’s presented in this paper? Is the first comparative analysis you did?
Jonathan Silvertown: Actually, it wasn’t the first piece of work. I did my PhD at the University of Sussex. One of my supervisors there was Paul Harvey, who was a zoologist, and one of the people who started the sort of new wave of comparative evolutionary biology using phylogenies. In my thesis, which was very eclectic and did a variety of different things, I looked at the evolution of masting in trees. We’re talking about 1980. I wasn’t using meta-analysis. I wasn’t even using phylogenetic analysis because it hadn’t really been developed at that point. But I was aware of the arguments that underlie those things. This was published in the Biological Journal of the Linnean Society in 1980. So, I’ve always had a sort of comparative perspective on things. Incidentally, that paper has been cited more than 700 times.
HS: What was the motivation for the work presented in the 1993 paper?
JT: At that time, I was visiting a laboratory in UNAM in Mexico City. I was teaching a class there on life history evolution in plants. I’m a plant population biologist. This field, as you’re no doubt aware, was founded by John Harper at Bangor in North Wales. I wasn’t a student of his, but I was very much influenced by him. He and John Maynard Smith at Sussex were the examiners on my PhD. That gives you a feeling for mixture of evolution and comparative approach and plant population biology that was influencing me at the time. I think this was in1980. I’d just done my PhD and moved to my first job at Open University. In Mexico, they were holding sort of a Darwinian celebration of plant population biology. It was being held there because somebody called José Sarukhán, who’s still around in Mexico, had done his PhD with John Harper in Bangor. He was a founder of plant population biology. He did a famous comparative study of demography of three species of buttercup in a field in Bangor. That study was very important. With Gadgil from India, he did a matrix model of the dynamics of those three buttercup species. That was absolutely pioneering. When Sarukhán went back to Mexico after his PhD, he was the director, I think, of the Instituto de Biología in Mexico City. Then he formed the Institute of Ecology in Mexico at UNAM. There was a whole sort of generation of ecologists from Mexico who came to Britain to do their PhDs. So, there were very strong ties between Mexico and Britain, in the field of ecology, in my generation and just before me. So, this meeting was organized and I somehow managed to get in there. I was only what 26 or 27 at the time. After that, I started going back to Mexico, funded by the British Council, to just teach a course at the university. And then, the British Council, who were paying for this, said, well, you really ought to start doing some research here. I then started talking to various people in the Institute of Ecology, which I think had just been founded. There was Miguel Franco, who you will have noticed I’ve written a lot with, and is a co-author of the 1993 paper. Miguel and I started talking about what we might do. He’d done his PhD at Bangor, as well, just like, Rodolfo Dirzo, who’s now an academician in the United States. I’ve always been interested in big and general patterns. I didn’t want to do just another study of another plant. And even then, in the late 1980s and early 1990s, it was beginning to be obvious that there were enough studies that you could start to try and draw out generalizations. Miguel and I discussed this, and then I went there in, I guess, 1992, and we sat down in a room in his office for about a week with a whole pile of papers and basically thought about how we could compare studies that were done in completely different ways on completely different plants. The concept of elasticity had just been published. And because elasticity sums to one across the matrix, and you can use this measure on any matrix, it was immediately obvious to me that this would be a perfect way of comparing literally, apples and oranges, if you like; things are very different. Then we needed a way of actually dividing up the elasticities into biologically meaningful components. Just looking at the matrix, we came up with those different areas of the matrix which we said approximated to different biological processes. And then we gathered them together into three variables, the G, L and F variables, and plotted them on a triangle. That’s the beginning of it all, the story of that paper.
HS: You said you first went there in 1980, and then continue to go back regularly since then. Was it usually to teach?
JS: Yes, I think I must have been there about four or five times, over 10 years or so.
HS: One of the papers you cite is a 1990 paper with Miguel Franco, which you also refer to for the data set. Was that the first paper you published with Miguel Franco?
JS: Let me look at that. It was quite a long time ago! Oh that, I call that a pot boiler. That was published in Evolutionary Trends in Plants, which was a rather obscure journal. That was sort of a report on what we were doing at the time, really.
HS: Did you and Miguel Franco put together all the studies you included in this paper after you decided to start this project?
JS: Yes.
HS: Did this happen in Mexico?
JS: Well, we started it there. Yeah, most of the work was done in Mexico over a fairly short period of time. Then, obviously, we had to work harder at it. I can’t remember exactly what happened. Certainly, that period, which would have been no more than a couple of weeks, was critical. That is when we worked out what we were going to do, and how we were going to do it. I think the data processing would have happened later, but we made a start there.
HS: How difficult this was it to put this database of studies together at that time, when accessing literature online was not so easy? Did you already know this literature well?
JS: Yes, I knew the literature pretty well, because it wasn’t nearly as big as it is now, for a start. I’d written my textbook Introduction to Plant Population Biology. So, I was pretty much on top of the literature in the field. Oh, the other thing I should mention is we decided to call the project Compadre, which in Spanish means mate or friend or partner, but it also stood for ‘comparative plant demographic research’. This was even before the EU started insisting on all research projects having an acronym. Coming up to date, there’s now a Compadre-2 being run by Roberto Salguero-Gómez in Oxford. In his very latest paper [published in 2019], he’s gathered together a database of about 650 matrices for plant populations, with a bunch of other people, including Miguel, but not including me. I’m doing other things these days. He has published a whole slew of papers. He calls his project Compadre, sort of out of recognition for the fact that Miguel and I started it off, which is nice of him. He’s also extending it now to animals, which is called Comadre. So there’s now a second life in this field. They’re using the same method we were using, obviously, with quite a lot of modern bells and whistles, to look at a whole range of different things, many of which we actually thought of looked at earlier on, but with much smaller sample sizes.
HS: You said you’ve now moved away from doing this kind of work. After this paper, for how long did you continue doing research along these lines?
JS: Well, off and on. Within the field of plant population biology, I’ve wandered about from one thing to another, because I have broad interests, and also because funding has been good in some areas and not in others, at different times. It’s good to have a portfolio of interests. I suppose, I was doing demography of some kind or other, right up until about 2000. I wouldn’t say I’ve completely left it behind. I’m still interested in life history evolution, fundamentally; that’s not demography.
HS: I noticed that you have a paper with Miguel Franco in 2004.
JS: This was a bee in the bonnet that Miguel had, which was that our original paper wasn’t actually measuring the right thing. It was based on elasticities of matrix elements and not vital rates. Miguel redid the analysis with a slightly bigger sample size – about 100 I think – and essentially came up with exactly the same results as we’d done earlier. This was a good thing, because it meant that our earlier results were sound.
HS: Who are the other two authors on the paper?
JS: Irene Pisanty was a plant ecologist in the science faculty at UNAM. She’s still there. Ana Mendoza was, I think, a postdoc at that point in UNAM. She’d also visited Bangor. She worked closely with José Sarukhán. A bit later, at various times, Irene and Ana and Miguel all came to the Open University where I was. In fact, all three of them came to the OU to work with me, after we published that 93 paper.
HS: So you continued to work with them after this paper?
JS: In various ways. Miguel has always been my main collaborator.
HS: What did the different authors bring to this paper?
JS: Miguel and I thought up the whole thing. I’ve already explained why I was thinking comparatively, and my interest in life histories and so on. I was the initiator and Miguel was very important in making it into a paper. I certainly don’t want to deny his important involvement. But I’m fairly sure the idea originally came from me. Irene and Ana helped us with the data processing and things like that.
HS: Did all the work happen between 1990 and 1993, mostly in Mexico?
JS: Well, um, Mexico and Britain. I was working on it back home as well. But, yeah, it came out of this Mexican connection.
HS: Did the four of you meet often during this work?
JS: Not very often. There was one occasion when all four of us were in Milton Keynes, but can’t remember exactly when that was. My reference point is something biological but not a plant. My first son was born in July of 92, and I was in Mexico in about March and thinking, I must get home. Irene Pisanty must have come over later that year before July, because I have in my mind a photograph of the whole family in our garden with Irene there and my wife very pregnant.
HS: Did all four of you meet in the UK?
JS: Yes. We knew each other already. I certainly knew Irene very well. She was really my first contact in Mexico. When I went to Mexico in 1980, I met Irene.
HS: How long did the writing take? Did you write the first draft?
JS: Yes, I did. It didn’t take very long, actually. I remember sending it to Journal of Ecology and not being at all sure how it was going to be received, because it was something quite different to anything they had published before. Jim White was the editor at the time. He’d been a student in Bangor, but he’d not really done demography as such. Not that that should make any difference; I don’t think it made any difference. But they published it without too much fuss, I seem to remember.
HS: Did you do most of the writing when you’re back in the UK?
JS: Uh, I honestly can’t remember. I would have done some of it there. I’m sure. But we would have finished it off between us after I’d gone back. To be quite honest, like, I can’t really remember that kind of detail. I’m sorry.
HS: Was Journal of Ecology the first place you submitted this to?
JS: Yes, it was.
HS: Do you remember whether it had an easy ride through peer review? Did it go through many rounds of review?
JS: No, It didn’t go through many rounds of review. One of the things that I was sensitive to, at the time, was I’d published a paper about the Park Grass Experiment, analyzing data from a long period of time – that’s another thing that came up with my PhD – I’d submitted that to a journal, and I’d been accused of fishing with statistics. I had to look up what that meant, at the time. Nowadays, most people will be familiar with that. But, anyway, it was a completely stupid thing for the referee to say, because they were blind to these interesting patterns, simply because I’d taken a lot of data and a lot of analysis and put the two together, which was pretty unusual back then. And, in essence, we were doing the same thing again in this paper, but in a totally different area, in demography. I was slightly worried, I’d been scarred, if you like, by this earlier experience. I think it was my very first paper, which was rejected by Journal of Ecology, and published by Journal of Applied Ecology. It has been cited a couple of hundred times. So, I was concerned that there might be a similar sort of criticism. And also, that it was just descriptive, and there was no experimental element or anything like that. I can’t remember how many papers I’d published at that time, but I was still comparatively young, and conscious of the weaknesses and things that might cause problems in publication. But, it was pretty smooth.
HS: How did you draw the figures in the paper?
JS: I think that matrix figure was drawn with pen and ink, and the hatching and so on with tones from Letraset, I expect.
HS: I’m guessing there weren’t any software with which you could make these figures?
JS: Um, well, the graph was drawn with software. This was the 90s. It wasn’t completely the dark ages. But computer output wasn’t necessarily that tidy. This was the era of dot matrix printers and things like that. So, you could actually get a better output by drawing. I’m looking now at Figure 2 and it looks like it’s drawn with ink. You can see the lines are not perfectly straight.
HS: Figure 3 also looks like it’s hand drawn.
JS: Actually, I think Figure 3 might be computer drawn. I think these were the days when you had a mixture of things. Things were changing.
HS: Could we go over the names of the people you’ve acknowledged to get a sense of who they were and how they helped?
JS: Sure. Ruben Perez-Ishiwara was a student and a lab assistant. Elena Alvarez-Buylla is quite a big name now in Evo-Devo. She was a young researcher in the Institute of Ecology in Mexico. Bastow Wilson was a friend of mine, who died fairly recently. He was at Otago in New Zealand. He would have provided the randomization tests. James Bullock was working with me as a postdoc. He’s at the UK Centre for Ecology & Hydrology, these days. Neal Enright was a friend of Miguel’s in Australia, who’d worked on matrices. Mike Gillman was a colleague of mine at the Open University. Jan van Groenendael was, I think, then at Waganingen, but now retired from Nijmegen in the Netherlands. He was one of the people on the original elasticity paper that kind of produced the method. Paul Harvey was my PhD supervisor from Sussex. Irene Ridge was a plant ecologist and a colleague of mine at the Open University. Tere Valverde was a PhD student of mine from Mexico. She was an excellent student and did lots of very good stuff.
HS: Do you remember how the paper was received when it was published? Did it attract attention? Was it considered controversial?
JS: Um, no, I don’t think it was considered controversial. Even in the best of times, it seems to me that things you publish can be quite slow to have an impact. With social media these days, of course, you would kind of expect Twitter to catch fire immediately if anything’s any good; but then it goes away just as quickly usually, if you’re so lucky as to attract any attention whatsoever. I think it was a while before this paper was noticed. I suppose, it’s probably regarded as something of a classic now; you probably wouldn’t be talking to me, otherwise. But what I do remember very distinctly is going to a meeting of the Ecological Society of America, at Madison, either just before or just after the paper was published, and giving a talk. The title of the talk was something like ‘Generality in plant demography’ or something like that. It was kind of promising that, there were patterns here that hadn’t been seen before and that were going to bring the field together, which I think, in a way, is correct. The room was absolutely packed. There were people standing at the door and trying to get in. It’s one of those occasions, when you think, Wow! Looking back on it, I realize that the room was actually quite small, but nonetheless, it was crowded. The talk, certainly, attracted a lot of a lot of interest. And something I was very aware of, at the time, was that we had produced a method that other people could use. This was a method that that could be easily deployed by other people. It was a way in which people doing demographic studies could compare their results with other people’s results. And that did begin to happen. I’m not sure how widely that’s actually been used. We used it a fair bit. There’s a paper in Conservation Biology using the method to look at conservation questions, and so on. And there’s another one about habitats and succession and things. I think we took it a fair way, and there were a number of papers, and, in fact, there still are a few papers, that use the triangle, but I think that things have gone beyond that now.
HS: In 1994 in the Journal of Ecology, Miguel Franco and you published a reply to a comment by Shea, Rees and Wood. Was that in relation to this 1993 paper?
JS: Let’s see, I can’t remember…Right. It’s really about tradeoffs. The big question is what can you infer from elasticities? One of the things we did in the 1993 paper, which I think was wrong, was we looked at correlations between elasticities and treated those as tradeoffs between the underlying traits. And Shea et al. pointed out that that, basically, was wrong. We accepted that, but said that these tradeoffs were not the main point of the paper, and that there are interesting patterns there, nonetheless. I think what this gets basically, is the difference between elasticities of matrix elements and elasticities of vital rates. This is sort of the thing that we tidied up later, in 2004; a long time later.
HS: You say, “matrix analysis yields the stable age or stage distribution and a vector of reproductive values. These are of interest in themselves (J. Silvertown & M. Franco, unpublished), and may also be used to calculate the elasticity eij of each element aij in the matrix.” Was this published subsequently?
JS: I don’t know whether we published anything on that. I mean, we were thinking about all sorts of ways in which we could use this database and these comparisons. It’s sort of slowly coming back to me that we were wondering whether there was something of interest in comparing stable stage distributions and so on, but that did not actually result in another publication.
HS: At the end of the paper, you say that this data set will be used to look at other topics in competitive plant demography. Did that happen?
JS: There were three or four, a lot. There was a paper about making a comparison with Philip Grimes’s CSR triangle, which he didn’t much like. I don’t know why he didn’t like it, because, actually, we were supporting him, but anyway. That was 92, so that was actually before this paper, but publication delays mean that things don’t all happen in sync, even these days. Then there was a paper in Plant Species Biology, which is a Japanese journal. I’d been asked to go and talk at a meeting in Japan, and I used our triangle approach to look at the relationship between habitat and demography, within species. That was in 93, as well. Then there was the paper in Conservation Biology in 1996, Interpretations of elasticity matrices as an aid to the management of plant populations. Then Miguel and I organized a Royal Society discussion meeting in 96, which was trying to introduce phylogeny into the mix. This was early days for that kind of thing.
HS: In this paper you cite Hal Caswell a few times.
JS: He is the grand daddy of matrices.
HS: Was his work a strong influence on your research at that point in time?
JS: Um, well, as much as he provided the theoretical underpinnings for it, yes. I know him now; I’m trying to remember if I knew him back then. Honestly, I can’t remember, but I certainly knew his work.
HS: Did this paper have any kind of direct impact on your career?
JS: Very difficult to say. I already had a tenured job. I had a research group. I remember thinking at the time that this was going to be a very important paper. I definitely thought that. I almost like to say I knew it, but you don’t know these things. I remember saying to Miguel in Mexico, this is going to be very important. And I think, for a time, it has been in certain respects. It’s the first time anybody has phoned me up from India to talk about it! I mean, my career – I don’t want to talk about in the past tense, but, obviously, a lot of it’s behind me- it’s been sort of eclectic. These days, I’m still doing research, but I’m heavily into writing books. On popular science, and things. I’ve moved institutions fairly recently and that’s opening up new opportunities. Um, I don’t know, I didn’t actually move from the Open University for decades. And it didn’t lead to my instant promotion, or grants or anything like that, really.
HS: I guess what I’m asking is whether you this paper or this piece of work became an important part of your identity?
JS: More by my textbook. I remember going to a job interview quite a few years ago, and being asked, what’s the most important thing you’ve published? I insisted it was my textbook. They obviously didn’t want that as an answer. They wanted me to pick a piece of research. People generally prioritize research over teaching and writing, but, I hate to say this, but that can just be snobbery. I think one can have influence in all sorts of ways. And my textbook, although now out of date – I haven’t written a new one for 10 years, and don’t intend to, by the way – has influenced tens of thousands of students, a handful of whom have gone on to become professional ecologists. But, you know, if you were to look back and say, what effect did I have? I think there’s still no question that the 30,000 copies of my textbook that sold in various editions have influenced people. I’ve even been told that by a few senior ecologists, younger than me, that that book was actually really important in their career. There are very few papers about which people will say that: I read the paper and it changed my life. But they sometimes say that about books.
HS: Do you have a sense of what this paper, mainly, gets cited for?
JS: I don’t even know what its current citation number is; it’s probably around 400 or something.
HS: It’s over 500 according to Google Scholar
JS: Well, my first question is, have those 500 people actually read it. It’s very handy to have a reference point. When you’re writing a paper you want some reference points. I’ve got quite a lot of papers that were written in that era, which are still cited as reference points. I’ve got one recently, actually, which is used as a reference point for anybody who wants to write about citizen science and which has been cited nearly 2,000 times. I’m a bit skeptical about what these numbers mean. I don’t think it necessarily means you’ve had that influence., I think it means people are fishing around, they want to signal that they know the field, and they are paying, in a sense, lip service. I’m glad for the citations; don’t get me wrong. But what does it actually mean? Um, it’s debatable.
HS: You said that this paper didn’t really have any direct impact on your career. In what ways did it shape the trajectory of your research subsequently?
JS: Before I answer that, I want to say, don’t get me wrong. I’m not saying this did me any harm or it didn’t contribute to my career. I’m only saying there are many different strands to that career, so it’s very difficult to factor out one thing versus another. Did it influence the course of my research? For a short while it did. It did influence the direction of my work for a while. But plant demography rather went out of fashion. And plant population biology, which Harper defined as a subject and I wrote a textbook about, for a while, was a sort of field unto itself. There’s still an Ecological Society of America special interest group on plant population biology. But my feeling is that ecology has become more integrated. And while there are certain things about plants and their population biology that are particular to plants, quite rightly, people now look at population biology in the broad sense, for patterns and rules and trends that cross all taxa, really. That’s very much grown. Like I mentioned, Rob Salguero-Gomez started off with plants but later started one on animals as well. It’s a natural progression.
HS: Today, 23 years later, would you say the main conclusions still hold true, more or less? If you were to redo the study today, what would you change?
JS: This was the first comparative paper of its kind. That was important. Obviously, the tradeoff between elasticities we interpreted rather simplistically; you could even say erroneously. We would not do that again, if we had to redo it. I think the method is still important; matrices are still the main way in which people analyze plant populations. You can use vital rates, but it turns out it doesn’t make much difference whether you do the more difficult math of looking at vital rates, or the easier approach of just looking at matrix components. I don’t have any regrets. I’m not really sure what I would do differently. I’m not sure how to answer that question.
HS: In a couple of places in the paper, you say that this approach, if successful, should help us understand the relationship between life history and the habitat templet. To what extent has that happened?
JS: There was a paper that I referred you to, looking explicitly at habitat and demography and saying, basically, under grazing and under succession and so on, plant populations move along this trajectory. And now, all this work that Rob and his colleagues in Compadre-2 are doing is, certainly, beginning to look at this Rob has got a database, now, that is 10 times the size of what we had. And then there are individual studies that have got, not just one matrix for one population, but 10 matrices for 10 populations, and so on. So, people have been able to look at space at space and time and species in more depth than we could. But I would say that people look at traits more than habitats these days. That’s the big focus at the moment.
HS: Would you say that the availability of more data has been a major step forward?
JS: Yes, I would. Obviously, there have been developments in technique and so on, in particular, the elasticity of vital rates as distinct from matrix elements – I keep coming back to that – and other methods as well, moving away from matrices altogether. But the availability of so many datasets has definitely been a major step forward.
HS: In the concluding lines of the paper you say, “This paper is only the first step towards a comparative demography of plants. We have demonstrated that the approach can reveal meaningful relationships between life history variables, between life history and life form and between life history and habitat, but there is still much to do. With a sample containing more species it would be possible to look for the influence of taxonomic constraints on demographic patterns. We are also acutely aware that demographic parameters for most species vary greatly in time and in space, and future studies should attempt to compare intraspecific patterns with interspecific ones.” From our conversation, am I right in understanding that a lot of this has happened through the work of people like Rob Salguero-Gomez…
JS: Not just Rob. A number of people have done this. Yvonne Buckley has done stuff on this. What we said there, has now been done. I wouldn’t say the last word has been written on it, but those things have been done. And I have to say they’ve been done by other people. I don’t know if that’s influence or what it is.
HS: Have you ever read this paper after it was published?
JS: Yeah, a couple of times, but it’s been a while.
HS: Do you remember in what context you went back to reading it?
JS: Um, well, when Shea et al. criticized it, I certainly would have gone back and checked, what did we actually say? But that’s a rather flippant answer. Um, have I revisited for inspiration, is that what you’re asking? Not really.
HS: I’m curious whether people go back to the papers they publish, or forget them, once they’re done?
JS: I haven’t forgotten about it. I published papers allied to this, for quite a few years afterwards. I sort of get what you’re trying to say, but I’ve published, I don’t know, 120 papers or something. Another way of asking this might be, which is the one I most often revisit? I don’t know the answer to that, having posed that question myself! I’m not embarrassed by it or anything. One just moves on. Yeah.
HS: Most interviewees, when asked this question, have said they only go back to it to check some detail, or when they use it in teaching.
JS: That’s exactly right. Because we’re having this conversation, I’ve looked at this paper right now, for the first time in decades!
HS: Would you could this paper as one of your favorites?
JS: I think it has to be. It was novel. I don’t know what influenced it’s had, but it was novel. Talking to you has been interesting, from my perspective, because it made me think back on the influences that were present on me at the time, that led me to do this. And I was conscious of those influences at the time, but I’ve not thought about them for a long time. With the benefit of hindsight, you can see, why me, why then, why that? Like I’ve described to you, the fact that I was working with Paul Harvey as my PhD supervisor led to my interest in comparative methods, I was influenced by John Harper in doing demography and so on. You put all of that together and you can see how it happened. That’s interesting to reflect upon. So, yes, it’s got to be up there among amongst them.
HS: Stepping back a bit, what got you interested in plants?
JS: Even further back, what got me interested in biology was I had an excellent biology teacher called Mr. Pond. I went as an undergraduate at University of Sussex and stayed on to do a PhD. I was interested in natural history, but why was I not interested in birds, particularly since most people are? I don’t know, really. I just took to plants, which is kind of unusual even today. I now have a big class of biology students here in Edinburgh. Another lecturer on the course is Dr Richard Milne, a plant biologist who is very inspirational. And students say things like, well, I always hated plants until I heard Richard’s lectures. We also have a very good botanic garden here in Edinburgh, and we take the students around that to enthuse them. I tell students that any fool can get interested in animals, but that appreciating plants requires subtle understanding. It’s a transparent ruse that doesn’t work, of course!
HS: What would you say to a student who’s about to read this paper today? Would you add any caveats they should keep in mind when reading it?
JS: The cautionary note is that it was written a long time ago. I won’t say, what are you reading that stuff for? I think it’s of historical interest. And that’s a good thing. But, it also means that there’s been an awful lot happened since. I certainly wouldn’t say, to a PhD student starting with me now, go and read this paper that I wrote in 1993. There’s definitely more recent work I would direct them to, such as our 2004 paper.
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