January 21, 2008

Probit output for ec902

probit.docHere is the output for the linear probability model and the probit model we saw in the classes today.

As mentioned, you can find problems when trying to open the excel dataset directly in Eviews. For some reason only half of it is actually imported. One way to get around that is to open the file in excel, then "save as" and change the file type to "comma delimited". Once you do that you should be able to properly open the file in eviews through the option "open foreign dataset" in the file menu. It worked for me at least.


November 24, 2007

ec902 – Output for consumption

There was a mistake in the output I showed in the classes for question 5 and 6 in problem set 5. More details here exc_sheet5-ec902.pdf(as usual, you need to sign in at the top right corner of this page to see the file).

November 16, 2007

ec902 – Restrictions apply

Here are a few notes on question 4 (which we could not fully cover in 2 classes) and question 5 in problem set 4. You need to log in to see the file. excercisesheet4.pdf

No much more to say really.


November 14, 2007

ec226 – The output and more

In this file ex4-bis.doc(you need to log in to see the file) you can find most of the eviews output for the questions in exercise sheet 4 that we already covered in the classes. 

The file also includes some comments that should help you go through the remaining questions in the same exercise sheet. We will definitely cover the remaining questions in the next class, but because that will only happen the day before your test you might want to make sure you work on these questions beforeyou come to classes.

As usual, feel free to contact me (especially if you spot mistakes) either by leaving a comment here or by emailing me.


November 07, 2007

ec226 – How to be robust.

(If you are an MSc student, you might be interested in how to use robust standard errors in eviews).

As mentioned in today's classes, we do not generally assume normality and homoskedasticity, so, provided we have a large sample, we employ a robust estimator for the variance of the OLS estimator which allows for heteroskedasticity.

The video below shows:

  • how to tell eviews that you want the robust standard errors.
  • how to obtain the correct test statistics for the hypothesis that all the slope coefficients are equal to zero (using the Wald option after estimation)

You should use the same command to carry out tests on a subset of coefficients as you are asked to do in excercise sheet 4. 

Keep in mind that the equation being estimated is:

%7Elog%28earnings_i%29%3D%5Calpha%2B%5Cbeta_1%7Es_i%7E%2B%5Cbeta_2%7Easvabc_i%7E%2B%7E%5Cvarepsilon_i%7E

If the video below does not appear (you need a Flash reader installed, click on PLAY, you should also be able to zoom-in by right-clicking on it) or it takes ages to upload, this pdf briefly guides you through the same things. robuststuff.pdf.

Feel free to ask if anything is not clear. I have now enabled anonymous comments.

Please, do try to solve the rest of excercise sheet 4 before the next class.


November 04, 2007

Can you hear that Vox?

Writing about web page http://www.voxeu.com/

I just wanted to point out this very interesting website which provides "research-based policy analysis and commentary from Europe's leading economists".

The site hosts articles by many prominent economists on a number of different topics. Although (or should I say luckily) the articles are short and non-technical, they contain references to research papers for those interested in finding out more about a specific topic.

Definitely interesting for a research student or researcher, but probably equally stimulating for undergraduates and postgraduates who might have this strange interest in the real world and who, at some point, we'll have to come out with some sort of research project.

I think it is worth keeping track of their RSS feeds so they will be on my side bar.


November 02, 2007

ec902– Should we get married?

The last bit of excercise sheet 2, which we could not cover in some classes, asked you to estimate this model.

ln%28w%29%3D%5Calpha%7E%2B%7E%5Cbeta_1%7Eln%28%5Cexp%29%7E%2B%7E%5Cbeta_2%7EFemale%7E%2B%7E%5Cbeta_3%7EMarried%7E%2B%7E%5Cbeta_4%7E%28Married*Female%29%7E%2B%7E%5Cvarepsilon%3CBR%3E

Note that  %26nbsp%3BMarried*Female%3D1%7Eif and only if the individual is BOTH female AND married.

How do we interpret %7E%5Cbeta_4%7E?

Well, we know that, ignoring whether a person is married or not, the effect of being female is the coefficient on the female binary variable, that is %7E%5Cbeta_2%7E. But if that person is also married then overall effect of being female becomes %7E%5Cbeta_2%7E%2B%7E%5Cbeta_4%7E.

Therefore, the ln(w) of a female differs by %7E%5Cbeta_2%7Efrom that of a man and the difference becomes %7E%5Cbeta_2%7E%2B%7E%5Cbeta_4%7Eif the female is also married.

Therefore %7E%5Cbeta_4%7Emeasures the additional effect of being married for a female. In other words, it measures whether the gender wage differential changes depending on whether the female is married or not.

Clearly, you can also interpret this focusing on the %26nbsp%3BMarried%7Evariable. In that case, the interpretation becomes: %7E%5Cbeta_4%7Emeasures the additional effect of being female for a married person. In other words, it measures whether the "marriage wage differential" changes depending on whether the married person is female or not.

Because our dependent variable is ln(w), the coefficient on these dummies can all be interpreted as approximate percentage changes. So, for example, suppose that we find that %7E%5Cbeta_2%7E, the coefficient on Female, is %26nbsp%3B-0.07%7E. That means that females on average earn approximately 7% %28%3D%7E%5Cbeta_2%7E*%7E100%29%7Eless than males.

Note: because BOTH the experience variable and the dependent variable are in logs, the coefficient %7E%5Cbeta_1%7Eon ln(exp) is an elasticity as we showed in the class. Therefore the interpretation is that when experience changes by 1%, the wage (our dependent variable) changes by %7E%5Cbeta_1%7E%25%7E. No need to transform the coefficient in this case.


November 01, 2007

ec226 – Equation editor

In 2 of the three classes yesterday I did not have time to go over how to use microsoft equation editor to type equations in Word.

As you know, you will have to submit your assignments electronically, so you need to familiarise yourself with this.

It is not particularly complicated. If you manage to reproduce the equation in this file equationeditor.doc, you should be ok.

Also, this webpage provides a simple guide (but that is probably more than you strictly need). Obviously, if you are having problems you can come and see me during my office hour or use this blog to interact.


October 31, 2007

Ok, let's try this.

You can read some scary stuff out thereon the potential (apparently mostly negative) impact that writing a blog can have on your (academic) job prospects. But that really seems to have to do with people writing about their personal lives, and that is not what I am up to.

Of course, there are plenty of happy economists with pretty secure jobs in the blogsphere. Interestingly, you can find peoplewho go as far as saying that blogs are the best way of learning economics, along with otherswho advice well established economists such as Mankiw that they should stop indulging in blogging as they have better things to do.

The latter post is actually pretty interesting as it summarises an on-going discussion on the future of economists in the blogsphere. One point raised is that given that a blog tends to take more and more of your time when you become serious about it, the best economists could end up getting out of the blogsphere due to the high opportunity cost they face. That, it is argued, could lead to a sort of lemon problem as the blogsphere would remain populated of low quality blogs.

Well, when reading that I couldn't help the feeling that my plan to start a blog was just tantamount to planting a lemon tree. But then, I really never intended to dispense my unsolicited economic advice to the world. The time will come for that, perhaps. For the time being, here is what I am going to do.

Teaching: I will use this blog to post any additional materials/comments I might have after the classes I teach in econometrics for the undergraduatesand the Msc students. There is obvious potential for some interaction with the students here and I'd be happy to experiment with that a bit. I mean, I answer emails anyway, so I guess there is no harm in making these exchanges available to the other students.

Research: I am not going to use this as some kind of research diary. I use my webpageon the department site to inform the world about the developments of my research when, of course, I think the time is right. Rather, I will occasionally use this blog to write short summaries/reviews of papers which might be only vaguely related to what I am currently working on. The focus will mainly be on applied microeconometrics papers and the stress will probably mostly be on how a general methodological/econometric problem is dealt with rather than on the specific research question the paper intends to address.

Ok, I hear you say, but why are you doing this? Well, I read these papers all the time and I find that writing down a few notes helps me a lot in improving my understanding if not of the solution, at least of the problem. So, if I blog these summaries/reviews that I write for myself anyway and nobody reads them, I guess I am not worse off. If somebody does read them, I might just happen to benefit from people's feedbacks and comments. I guess a blog can be one way of following Avinash Dixit'sadvice (he is a theorist, of course, but mutatis mutandis...):

Many ideas, and techniques for theorizing, will come to you by accident. But don't wait for such accidents to happen; encourage them. Always be on the lookout for examples, questions etc that relate to what you are doing, or something you worked on once but set aside. A newspaper article or a current affairs program or a chance remark by a colleague can get you started. A totally unrelated theoretical article may use a technique that proves useful for your problem, and gets restarted on something that had stalled.

Dixit, My System of Work (Not!),The American Economist, Spring 1994.

Of course, the next point in Dixit's paper is:

Learn to manage your time

I better go now.


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