# Why don't economists do regression diagnostics?

There is a lot of talk about regression diagnostics in tutorials on the web, but then in economics research papers nobody actually reports residual plots, collinearity checks etc. Is there any reason for this?

• Thats a good question. I assume many do, but fail to show the parameters of their diagnostics in their papers. This also boils down to the fact that many economics papers are not reproducible. There have been several discussions on this topic in economics and other fields of science as well. Jan 26 at 14:19

Why don't economists do regression diagnostics?

People do regression diagnostics. I don't know of any respectable researcher that would not perform regression diagnostics and virtually any paper will have hints that the regression diagnostics was performed. For example, in tables with regression results you will find remarks on White or HAC errors being used to either correct for heteroskedasticity or autocorrelation or both, or references to corrections for cross-sectional dependence etc.

Only very unscrupulous scholars would make claims about these issues being/not being present without some testing. So scientists (or at least the good ones) always perform regression diagnostics.

To address the question in the body:

in economics research papers nobody actually reports residual plots, collinearity checks etc. Is there any reason for this?

Yes, almost all scientific journals have very strict page limit of between 30-60 pages with most journals having page limit around 40 pages. In addition shorter articles are often more preferred and attract wider readership because people usually prefer to read shorter papers. Also, note that page limits are usually inclusive of list of references which often can eat up another 1-5 pages and also all other stuff. Only online appendices are excluded from page limit.

Now documentation of regression diagnostic can easily eat up 10 pages if you want to do it properly with all the plots (or even more). Moreover, regression diagnostic is not of much interest in itself. You preform it in order to know how to properly specify your model or what identification strategy to use. Once you figure that you just use the appropriate model, so by itself regression diagnostic has little value for reader as it carries very little information about research result. As mentioned in the first part of the answers people will still mention in their paper that there was autocorrelation or heteroskedasticity and how they corrected for it (and so on for other problems), so there is not that much point in additionally wasting space in the paper on showcasing all the auxiliary diagnostics. Any mistrustful researchers can just request their data and rerun that diagnostic themselves.

Consequently, the reason for that is simply that there is not enough space for it in the paper, and because you always have to economize on the space given. If you would really want to do it, it would usually end up eating 1/3 of a precious space, You would be surprised how common problem it is for researchers to actually fit their research in the page limit. Often you will be forced to relegate even main derivations to online appendices just to fit in the limit. In the end anything that is not of great importance to support or interpret the main result will simply not make the cut.

As mentioned in the comments this can sometimes cause issues with reproducibility, but nowadays the solution to that is that journals require scholars to post their code that was used to derive results (where you would find also regression diagnostics) rather than actually report it in the paper, for the reasons mentioned above.

• Can you point to a paper where code actually does a couple of diagnostics of their main results (and I don’t mean running different specifications as robustness checks)? Jan 26 at 15:50
• also, in Burde and Linden (2013) Bringing Education to Afghan Girls: A Randomized Controlled Trial of Village-Based Schools, they actually even use regression analysis and in their dofiles you can see residual diagnostic and even unadjusted results from auxiliary regressions before the main published results.
– 1muflon1
Jan 26 at 16:31
• Thanks! I would not count placebo tests as diagnostic but as robustness checks. The residual and outlier checks in Burde and Linde definitely count, but even for them there would still be checks for linearity, misspecification, and multicollinearity to be done if one would follow a checklist like approach as in the tutorials: stats.idre.ucla.edu/stata/webbooks/reg/chapter2/… It doesn't seem to me that in practice researchers follow such a strict diagnostic procedure and I wanted to understand why, not criticize it Jan 26 at 16:58
• @Papayapap I don’t think those placebo tests count as robustness checks. They test validity (they are falsification tests) of the result not how robust the estimates are. Also, it is often not necessary to do specific tests for multicollinearity as that shows in the reg results. If you don’t see unusually large variances you can skip that. Next, in this case they were using randomized control trials with large number of controls so OVB is not big concern there, finally non-linearity would show in their residual plots as with multicollinearity that can be tested further if
– 1muflon1
Jan 26 at 17:13
• @1muflon OK. So you took "do" as important, and I took "reports" as important. I get it now. That makes sense. Thanks for clarification. Feb 25 at 2:22

This is a very thoughtful question.

I think it is related with (i) the purpose and (ii) the sample size. Econometrics is very often concerned with causality (rather than prediction or forecast). For causality, correct model specification, consistency, and valid standard errors are important. Things such as multicollinearity (high correlation), non-normality, etc., are irrelevant (especially with large data sets).

For example, multicollinearity typically leads to large standard errors but, importantly, no biases. If you drop some variables due to multicollinearity, it just means you fail to control for the variables you initially intended to control for; that is, your estimator is biased. Nonnormality check (e.g., normal Q-Q plot) is no important as long as the sample size is large due to the central limit theorem. Outliers are data points just like any other; who gave you the authority to omit them at your will? By dropping the 'outliers' you are just restricting the population in a fancy way; you only get the criticism that your estimator is biased. VIF? If you drop variables due to high VIF, it means you have an inconsistent (biased) estimator.

Selecting a model based on data is dangerous. It will be hard for you to defend your model chosen by lasso if you want to say something about causal effects unless you experiment with bleeding edge econometric techniques. Inferences are to be done for a given model (created by your thought), not a model suggested by statistics (i.e., by a computer).

These days we don't even care much about testing heteroskedasticity becase the sample size is large and we can always do HC inferences. Autocorrelation is not an isssue, as it only complicates standard errors, which you can fix by HAC.

If you are interested in prediction/forecast, those things might be useful more. But even for that, the said diagnostics are too old-fashioned. People have already moved on to lasso and other machine learning techniques. I think the said diagnostics might survive in (non-econometric) textbooks and tutorials, but will die out eventually in econometric practices. But if you have small samples, the story is different. It also happens that old things are found useful in completely different contexts. For example, IF's are very useful for computing standard errors.

• There are some issues with the answer, "Nonnormality check (e.g., normal Q-Q plot) is no important as long as the sample size is large due to the central limit theorem." - No this is common misconception, but this is not what central theorem says. Central theorem is about asymptotic distribution, and sure often in large samples you will get approx normal distribution but not always. In fact there are whole classes of models where normality even in regression with high number of observation will be violated (e.g. EG-cointegration model). Furthermore, errors will likely be non-normal if you
– 1muflon1
Feb 21 at 18:42
• (i) Error normality has nothing to do with consistency. (ii) If we are not very fussy (cases the Lindeberg conditions violated), CLT can be applied and the estimator is asymptotically normal, and thus testing based on t-stats is asymptotically valid. (iii) Log is a completely different issue. It's about model specification. (iv) Bad performance of HAC inferences can be practically important, but in fact there are not many cases where HAC is relevant in cross sectional analyses. (In panel, cluster.) (v) Time series is totally different. I focused on cross sections (and panel data). Feb 22 at 1:32
• (vi) Cointegration is a time-series topic. I'm not talking about it. I don't think OP is talking about it. Feb 22 at 1:46
• But it is generally agreed by many authors in the literature, you should test for normality even if your $n<200$ or some authors even argue $n<500$ or even more. Once you get to dataset with thousands of observations it will not be an issue (assuming model is well specified), but claiming that it’s not an issue generally is not appropriate. 3. It is connected to this issue since model mispecification shows up in the errors. You can use some normality tests as quick tests for proper specification before doing some more serious testing. 4. Sure, that’s why I said in marginal cases but that
– 1muflon1
Feb 22 at 2:06
• Still means accepting unnecessary mistakes and for what? To save 15-30 minutes on heteroskedasticity/autocorrelation testing? That is just simply unprofessional in my opinion. 5. You never mention that in your answer and panels with long $T$ have similar issues that pure time series has. There are actually even more things to test such as cross-sectional dependence which is serious issue. 6. OP never states that they are interested just in cross-section or panel data
– 1muflon1
Feb 22 at 2:13