Cochrane

Here is an abridged version of the article that Vince summarizes in his slides.


Writing Tips for Ph.D. Students

John H. Cochrane

Graduate School of Business

University of Chicago

I. ORGANIZATION

Figure out the one central and novel contribution of your paper. Write this down in one paragraph. As with all your writing, this must be concrete. Don’t write, “I analyzed data on executive compensation and found many interesting results.” Explain what the central results are. For example, Fama and French 1992 start their abstract with: “Two easily measured variables, size and book-to-market equity, combine to capture the cross-sectional variation in average stock returns associated with market β, size, leverage, book-to-market equity, and earnings-price ratios.”

Distilling your one central contribution will take some thought. It will cause some pain, because you will start to realize how much you’re going to have to throw out. Once you do it, though, you’re in a much better position to focus the paper on that one contribution, and help readers to get it quickly.

Your readers are busy and impatient. No reader will ever read the whole thing from start to finish. Readers skim. You have to make it easy for them to skim. Most readers want to know your basic result. Only a few care how it is different from others. Only a few care if it holds up with different variable definitions, different instrument sets, etc. Organize the paper in “triangular” or “newspaper” style, not in “joke” or “novel” style. Notice how newspapers start with the most important part, then fill in background later for the readers who kept going and want more details. A good joke or a mystery novel has a long windup to the final punchline. Don’t write papers like that — put the punchline right up front and then slowly explain the joke. Readers don’t stick around to find the punchline in Table 12.

(Most writers) get this exactly wrong, and we never really find out what the contribution of the paper is until the last page, the last table, and the last 5 minutes of the seminar. A good paper is not a travelogue of your search process. We don’t care how you came to figure out the right answer. We don’t care about the hundreds of things you tried that did not work. Save it for your memoirs.

Abstract

Most journals allow 100-150 words. Obey this limit now. The main function of the abstract is to communicate the one central and novel contribution, which you just figured out. You should not mention other literature in the abstract. Like everything else, the abstract must be concrete. Say what you find, not what you look for. Don’t write, “data are analyzed, theorems are proved, discussion is made.”

Introduction

The introduction should start with what you do in this paper, the major contribution. You must explain that contribution so that people can understand it. Don’t just state your conclusion: “My results show that the pecking-order theory is rejected.” Give the fact behind that result. “In a regression of x on y, controlling for z, the coefficient is q.”

The first sentence is the hardest. Do not start with philosophy, “Financial economists have long wondered if markets are efficient.” Do not start with “The finance literature has long been interested in x.” Your paper must be interesting on its own, and not just because lots of other people wasted space on the subject. Do not start with a long motivation of how important the issue is to public policy. All of this is known to writers as “clearing your throat.” It’s a waste of space. Start with your central contribution.

Three pages is a good upper limit for the introduction.

I don’t write a “roadmap” paragraph: “Section 2 sets out the model, section 3 discusses identification, section 4 gives the main results, section 5 checks for robustness, section 6 concludes.” It seems a waste of space; readers will figure it out when they get there and I save a paragraph against the editor’s page count. Make your own mind up about this question; but realize it’s not mandatory.

Literature review

Do not start your introduction with a page and a half of other literature. First, your readers are most interested in just figuring out what you do. They can’t start wondering if it’s better than what others have done until they understand what you do. Second, most readers do not know the literature. It’s going to be hard enough to explain your paper in simple terms; good luck explaining everyone else’s too.

After you’ve explained your contribution, then you can write a brief literature review. Make it a separate section of otherwise set it off so disinterested people can skip it. Remember, it will be very hard for people to understand how your paper is different from others’ given that they don’t understand your paper yet, and most of them have not read the other papers.

Be generous in your citations. You do not have to say that everyone else did it all wrong for your approach and improvements to be interesting.

The main point of the literature review should be to set your paper off against the 2 or 3 closest current papers, and to give proper credit to people who deserve priority for things that might otherwise seem new in your paper. Some people worry a lot about strategic citations; choosing citations to hint to editors who they should assign as referees and adding loads of citations to make sure referees see themselves.

Body of the paper

Your task now is to get to the central result as fast as possible. Most papers do precisely the opposite: They have a long motivation, a long literature review, a big complex model that then gets ignored, descriptive statistics, preliminary results, a side discussion or two and then finally Table 12 of “main estimates.” By then, we’re all asleep.

Here’s the rule: There should be nothing before the main result that a reader does not need to know in order to understand the main result.

Conclusions

Conclusions should be short and sweet. Do not restate all of your findings. One statement in the abstract, one in the introduction and once more in the body of the text should be enough! You can include a short paragraph or two acknowledging limitations, suggesting implications beyond those in the paper. Keep it short though — don’t write your grant application here outlining all of your plans for future research. And don’t speculate; the reader wants to know your facts not your opinions.

Appendices

Appendices are a great tool. Take that delicious section that has so many insightful comments on the literature, the general version of the model, the 57 robustness exercises that you did, and dump them in to an appendix. This is a good way to get them out of the paper. Eventually you’ll dump them out of the appendix too.

Seriously, careful authors, referees and critics often want to document that the main result is robust to various other ways of doing things. You have to do that, but once you’ve verified that it does not make that much difference and you’ve found the one best way of doing things in your main result, it isn’t worth space in the paper to present all the checks and variations. Appendices are a great way to solve this problem, and you can just summarize all the things you did in the paper. You can put the appendix on your and the journal’s website.

II. WRITING

Keep it short Keep the paper as short as possible. Every word must count. As you edit the paper, ask yourself constantly, “can I make the same point in less space?” and “Do I really have to say this?” Final papers should be no more than 40 pages. Drafts should be shorter. (Do as I say, not as I do!) Shorter is better.

Don’t repeat things. In other words, if you’ve said it once, you don’t have to say it again. Most of all, it uses up extra space and reader’s patience to have to see the same point made over and over again. So, once again, repetition is really a bad idea. (Get the picture?) “In other words” is a sign of trouble. Go back and say it once, right.

General points

Follow the rule “first describe what you do, then explain it, compare it to alternatives, and compare it to others’ procedures” at the micro level as well as the macro level. For example, in describing a data transformation, just start with, say, “I adjust income by the square root of household size”. Then tell us why adjusting is important, and then talk about different adjustment functions. Most writers do all this in the reverse order.

Previews and recalls convey poor organization. “As we will see in Table 6” “Recall from section 2” “this result previews the extra analysis of section 4” all often mean you didn’t put things in the right order.

Strive for precision. Read each sentence carefully. Does each sentence say something, and does it mean what it says?

Document your work. A fellow graduate student must be able to sit down with your paper and all alone reproduce every number in it from instructions given in the paper, and any print or web appendices. The usual student paper falls short here. There is a sea of verbiage, but I can’t figure out how the central table of results was computed, how standard errors were computed, how a simulation was conducted, etc.

Simple is better. Most students think they have to dress up a paper to look impressive. The exact opposite is true: The less math used, the better. The simpler the estimation technique, the better.

Footnotes

Don’t use footnotes for parenthetical comments. If it’s important, put it in the text. If it’s not important, delete it. Parenthetical comments in footnotes usually mean you haven’t organized your ideas; you haven’t figured out where to put this thought in a proper linear sequence. Do you really want the reader to stop and read this? Then it should be in the text. Do you think the average reader should not stop? Then delete the footnote. Obviously, lots of parentheses are just as bad as lots of footnotes.

Use footnotes only for things that the typical reader genuinely can skip, but a few readers might want to have attached to the current point. Long lists of references, simple bits of algebra, or other documentation are good candidates for footnotes.

Tables

Each table should have a self-contained caption so that a skimming reader can understand the fact presented without having to go searching through the text for things like the definitions of Greek letters. Don’t go nuts here; some captions are longer than the paper. In my opinion, you can leave out details of variable construction and similar items. “Book/market ratio” is fine; you don’t have to tell me that you got book values in June from Compustat. The goal is to allow a skimming reader to understand the table, not to substitute for the detailed documentation that must be in the paper somewhere.

No number should appear in a table that is not discussed in the text. You don’t have to mention each number separately; “Row 1 of Table 3 shows a u-shaped pattern” is ok. “Table 5 shows summary statistics” (period) is not ok. If it’s not worth writing about in the text, it’s not worth putting in the table.

Use the correct number of significant digits, not whatever the program spits out. 4.56783 with a standard error of 0.6789 should be 4.6 with a standard error of 0.7. Two to three significant digits are plenty for almost all economics and finance applications.

Use sensible units. Percentages are good. If you can report a number as 2.3 rather than 0.0000023, that’s usually easier to understand.

Figures

Good figures really make a paper come alive, and they communicate patterns in the data much better than big tables of numbers. Bad or poorly chosen figures waste a lot of space. Again, give a self-contained caption, including a verbal definition of each symbol on the graphs. Label the axes. Use sensible units. Don’t use dotted line types that are invisible when reproduced. Don’t use dashes for very volatile series.

Writing tips

1. The most important thing in writing is to keep track of what your reader knows and doesn’t know. Most Ph.D. students assume far too much. No, we do not have the details of every paper ever written in our heads. Keep in mind what you have explained and what you have not.
2. The reader usually wants most of all to understand your basic point, and won’t start criticizing it before he or she understands it. That’s behind my advice to first state and explain what you do, and save defending it and comparing it to other approaches until much later.
3. Use active tense. Not: “it is assumed that τ = 3”, “data were constructed as follows.” Gee, I wonder who did that assuming and constructing? Search for “is” and “are” in the document to root out every single passive sentence.
4. “I” is fine. Don’t use the royal “we” on a sole-authored paper. “I assume that τ = 3.” “I construct the data as follows.” If it seems like too much “I,” you can often avoid the article altogether. For example, I think it’s ok to write “Table 5 presents estimates” rather than “I present estimates in Table 5”, though a purist might object to making a Table the subject of a sentence. I use “we” to mean “you (the reader) and I,” and “you” for the reader. “We can see the u-shaped coefficients in Table 5” or “You can see the u-shaped coefficients” is much better than “The u-shaped coefficients can be seen” (passive) or “one can see the u-shaped coefficients” (who, exactly?)
5. Much bad writing comes down to trying to avoid responsibility for what you’re saying. That’s why people resort to passive sentences, “it should be noted that”, poor organization with literature first and your idea last, and so on. Take a deep breath, and take responsibility for what you’re writing.
6. Present tense is usually best. You can say “Fama and French 1993 find that” even though 1993 was a while ago. The same goes for your own paper; describe what you find in Table 5 not what you will find in Table 5. Most importantly, though, keep the tense consistent. Don’t start a paragraph in past tense and finish it in the future.
7. Use the normal sentence structure: subject, verb, object. Not: “The insurance mechanisms that agents utilize to smooth consumption in the face of transitory earnings fluctuations are diverse” Instead: “People use a variety of insurance mechanisms to smooth consumption.” (I also changed the starchy “agents” to the concrete “people,” and the simple “variety” rather than the fancy “diverse.” Actually, this whole sentence probably should be dumped; it was introducing a paragraph that described the mechanisms. It’s a throat-clearing sentence that violates the rule that every sentence should mean something. The fact that people use a variety of mechanisms is not big news; the news is what the mechanisms are.)
8. Avoid technical jargon wherever possible.
9. Writing should be concrete, not abstract. (Insert concrete examples.)

Little writing tips

1. Don’t use adjectives to describe your work: “striking results” “very significant” coefficients, etc. If the work merits adjectives, the world will give them to you.
2. If you must use adjectives, don’t use double adjectives. Results are certainly not “very novel.”
3. Use simple short words not big fancy words. “Use” not “utilize.” “Several” not “diverse”.
4. It is usually the case that most good writers find that everything before the “that” should be deleted from a sentence. Read that sentence again starting at “Everything”: it’s true, isn’t it? “It should be noted that” is particularly obnoxious. Just say what you want to say. “It is easy to show that” means that it isn’t. Search for “that” in the document to get rid of these. Similarly, strike “A comment is in order at this point.” Just make the comment. These phrases also violate the rule that each sentence should mean what it says. Is the point of the sentence really that “it should be noted?” Or is this just a wimpy way to bring up the topic?
5. Clothe the naked “this.” “This shows that markets really are irrational...” This what? “This” should always have something following it. “This regression shows that....” is fine. More generally, this helps (no, that should be “this rule helps,” right?) you to avoid an unclear antecedent to the “this.” Often there are three or more things in recent memory that “this” could point to.
6. Hyphens are widely misused. Here’s the rule from the JFE style sheet: “Hyphens are used for true compound modifiers before the noun (e.g., after-tax income, risk-free rate, two-day return, three-digit SIC code, value-weighted index) unless part of the compound modifier is an adverb ending in ‘ly’ (e.g., previously acquired subsidiary, equally weighted index, publicly traded stock). When there is no risk of misinterpretation, the hyphen can be omitted, but the treatment must be consistent throughout the paper.” Note the hyphen is optional, so you don’t have to construct monstrosities like “continually-rebalanced-equally-weighted portfolio.” Don’t use hyphens in other circumstances, e.g. “The paper focuses on small-stocks.”
7. Strike “I leave x for future research.” We’re less interested in your plans and excuses than we are in your memoirs.
8. Keep down the number of clauses in your sentences, and the number of things kept hanging.
9. “Where” refers to a place. “In which” refers to a model. Don’t write “models where consumers have uninsured shocks,” write “models in which consumers have uninsured shocks.”
10. Don’t abbreviate authors’ names, “FF show that size really does matter.” There is always enough space to spell out people’s names. You’d want them to write out yours, no?
11. It is appropriate to thank people who have helped you in the author footnote. I don’t add the qualifier about not blaming people I thank for comments for mistakes. It goes without saying. I don’t list every single place I’ve given the workshop in the thanks. I’m not ungrateful, but the long list can get out of hand.
12. Don’t start your paper with a cute quotation.
13. Don’t overuse italics. (I use them far too much.) It’s best to use them only when the emphasis in a sentence would otherwise not be clear — but maybe then you should rewrite the sentence so that the emphasis really is clear. (Who is that shouting in here?)
14. When describing the sign of a casual link, one direction is enough. “When Jane goes up (down) on the teeter-totter, Billy goes down (up) on the other side,” the stuff in the parentheses is distracting. Add “and vice versa” if you must.
15. Every sentence should have a subject, verb and object. No sentences like “No sentences like this.”

Conclusion

Many economists falsely think of themselves as scientists who just “write up” research. We are not; we are primarily writers. Economics and finance papers are essays. Most good economists spend at least 50% of the time they put into any project on writing. For me, it’s more like 80%.

Pay attention to the writing in papers you read, and notice the style adopted by authors you admire.

I got a lot out of reading William Zinsser’s On Writing Well l, and D. McCloskey’s Rhetoric of Economics. I also found Glenn Ellison’s “The slowdown of the economics publishing process” in the JPE useful for thinking about how papers should be structured (and refereed and edited, but that’s another story).

VINCE's NOTE

I omitted the following two sections from Cochrane's original paper

• Tips for empirical work (please see p. 9 of pdf @ http://bit.ly/CochranePhD)
• Seminar presentations (please see p. 11 of pdf @ http://bit.ly/CochranePhD)