I have consulted for Microsoft Corporation, Facebook, Amazon, and Lilly Corporation. In this case the clustering adjustment is justified by the fact that there are clusters in the population that we do not see in the sample. White standard errors (with no clustering) had a simulation standard deviation of 1.4%, and single-clustered standard errors had simulation standard deviations of 2.6%, whether clustering was done by firm or time. If clustering matters it should be done, and if it does not matter it does no harm. Typically, the motivation given for the clustering adjustments is that unobserved components in outcomes for units within clusters are correlated. She therefore assigns teachers in "treated" classrooms to try this new technique, while leaving "control" classrooms unaffected. BibTex; Full citation; Publisher: National Bureau of Economic Research Year: 2017. 50,000 should not be a problem. local labor markets, so you should cluster your standard errors by state or village.” 2 Referee 2 argues “The wage residual is likely to be correlated for people working in the same industry, so you should cluster your standard errors by industry” 3 Referee 3 argues that “the wage residual is … Second, in general, the standard Liang-Zeger clustering adjustment is conservative unless one of three conditions holds: (i) there is no heterogeneity in treatment effects; (ii) we observe only a few clusters from a large population of clusters; or (iii) a vanishing fraction of units in each cluster is sampled, e.g. For example, replicating a dataset 100 times should not increase the precision of parameter estimates. Tons of papers, including mine, cluster by state in state-year panel regressions. Phil, I’m glad this post is useful. 366 Galvez Street In empirical work in economics it is common to report standard errors that account for clustering of units. Am I correct in understanding that if you include fixed effects, you should not be clustering at that level? settings default standard errors can greatly overstate estimator precision. Typically, the motivation given for the clustering adjustments is that unobserved components in outcomes for units within clusters are correlated. Adjusting standard errors for clustering can be important. The site also provides the modified summary function for both one- and two-way clustering. Clustered Standard Errors 1. Accurate standard errors are a fundamental component of statistical inference. In empirical work in economics it is common to report standard errors that account for clustering of units. The technical term for this clustering, and adjusting the standard errors to allow for clustering is the clustering correction. Typically, the motivation given for the clustering adjustments is that unobserved components in outcomes for units within clusters are correlated. With fixed effects, a main reason to cluster is you have heterogeneity in treatment effects across the clusters. Abstract. Cite . 1. However, because correlation may occur across more than one dimension, this motivation makes it difficult to justify why researchers … To adjust the standard errors for clustering, you would use TYPE=COMPLEX; with CLUSTER = psu. You want to say something about the association between schooling and wages in a particular population, and are using a random sample of workers from this population. In empirical work in economics it is common to report standard errors that account for clustering of units. Regarding your questions: 1) Yes, if you adjust the variance-covariance matrix for clustering then the standard errors and test statistics (t-stat and p-values) reported by summary will not be correct (but the point estimates are the same). Therefore, If you have CSEs in your data (which in turn produce inaccurate SEs), you should make adjustments for the clustering before running any further analysis on the data. It’s easier to answer the question more generally. You can handle strata by including the strata variables as covariates or using them as grouping variables. We are grateful to seminar audiences at the 2016 NBER Labor Studies meeting, CEMMAP, Chicago, Brown University, the Harvard-MIT Econometrics seminar, Ca' Foscari University of Venice, the California Econometrics Conference, the Erasmus University Rotterdam, and Stanford University. at most one unit is sampled per cluster. In empirical work in economics it is common to report standard errors that account for clustering of units. This perspective allows us to shed new light on three questions: (i) when should one adjust the standard errors for clustering, (ii) when is the conventional adjustment for clustering appropriate, and (iii) when does the conventional adjustment of the standard errors matter. 2. We take the view that this second perspective best fits the typical setting in economics where clustering adjustments are used. In empirical work in economics it is common to report standard errors that account for clustering of units. In addition to working papers, the NBER disseminates affiliates’ latest findings through a range of free periodicals — the NBER Reporter, the NBER Digest, the Bulletin on Retirement and Disability, and the Bulletin on Health — as well as online conference reports, video lectures, and interviews. ^^with small clusters, clustered errors are smaller than they should be, but on average are much larger than OLS errors. In empirical work in economics it is common to report standard errors that account for clustering of units. Clustered standard errors are often useful when treatment is assigned at the level of a cluster instead of at the individual level. The Moulton Factor provides a good intuition of when the CRVE errors can be small. The topic of heteroscedasticity-consistent (HC) standard errors arises in statistics and econometrics in the context of linear regression and time series analysis.These are also known as Eicker–Huber–White standard errors (also Huber–White standard errors or White standard errors), to recognize the contributions of Friedhelm Eicker, Peter J. Huber, and Halbert White. If you are running a straight-forward probit model, then you can use clustered standard errors (where the clusters are the firms). When Should You Adjust Standard Errors for Clustering? In empirical work in economics it is common to report standard errors that account for clustering of units. Typically, the motivation given for the clustering adjustments is that unobserved components in outcomes for units within clusters are correlated. This is standard in many empirical papers. 10 / 24 Misconception 2: If clustering matters, one should cluster There is also a common view that there is no harm, at least in large samples, to adjusting the standard errors for clustering. Then you might as well aggregate and run … All Rights Reserved. In some experiments with few clusters andwithin cluster correlation have 5% rejection frequencies of 20% for CRVE, but 40-50% for OLS. In empirical work in economics it is common to report standard errors that account for clustering of units. Phone: 650-725-1874, Learn more about how your support makes a difference or make a gift now, SIEPR envisions a future where policies are underpinned by sound economic principles and generate measurable improvements in the lives of all people.  Read more, Stanford University   |   © 2020 Stanford Institute for Economic Policy Research, By  Alberto Abadie, Susan Athey, Guido W. Imbens, Jeffrey Wooldridge, Stanford Institute for Economic Policy Research. Typically, the motivation given for the clustering adjustments is that unobserved components in outcomes for units within clusters are correlated. When you are using the robust cluster variance estimator, it’s still important for the specification of the model to be reasonable—so that the model has a reasonable interpretation and yields good predictions—even though the robust cluster variance estimator is robust to misspecification and within-cluster correlation. However, because correlation may occur across more than one dimension, this motivation makes it difficult to justify why researchers use clustering in some dimensions, such as geographic, but not others, such as age cohorts or gender. When Should You Adjust Standard Errors for Clustering? The 2020 Martin Feldstein Lecture: Journey Across a Century of Women, Summer Institute 2020 Methods Lectures: Differential Privacy for Economists, The Bulletin on Retirement and Disability, Productivity, Innovation, and Entrepreneurship, Conference on Econometrics and Mathematical Economics, Conference on Research in Income and Wealth, Improving Health Outcomes for an Aging Population, Measuring the Clinical and Economic Outcomes Associated with Delivery Systems, Retirement and Disability Research Center, The Roybal Center for Behavior Change in Health, Training Program in Aging and Health Economics, Transportation Economics in the 21st Century. I If nested (e.g., classroom and school district), you should cluster at the highest level of aggregation I If not nested (e.g., time and space), you can: 1 Include fixed-eects in one dimension and cluster in the other one. Typically, the motivation given for the clustering adjustments is that unobserved components in outcomes for units within clusters … When Should You Adjust Standard Errors for Clustering? However, because correlation may occur across more than one dimension, this motivation makes it difficult to justify why researchers … By Alberto Abadie, Susan Athey, Guido Imbens and Jeffrey Wooldridge. Third, the (positive) bias from standard clustering adjustments can be corrected if all clusters are included in the sample … THE Health Secretary told Brits in Tier 4 to “act as if you have the virus” after Boris Johnson cancelled Christmas for millions in the South East. We outline the basic method as well as many complications that can arise in practice. This motivation also makes it difficult to explain why one should not cluster with data from a randomized experiment. In empirical work in economics it is common to report standard errors that account for clustering of units. The questions addressed in this paper partly originated in discussions with Gary Chamberlain. Clustering is an experimental design issue if the assignment is correlated within the clusters. lm.object <- lm(y ~ x, data = data) summary(lm.object, cluster=c("c")) There's an excellent post on clustering within the lm framework. We take the view that this second perspective best fits the typical setting in economics where clustering adjustments are used. This week Northern Ireland announced six-weeks of full lockdown, while Wales ann… In this paper, we argue that clustering is in essence a design problem, either a sampling design or an experimental design issue. Adjusting for Clustered Standard Errors. However, performing this procedure with the IID assumption will actually do this. It is a sampling design issue if sampling follows a two stage process where in the first stage, a subset of clusters were sampled randomly from a population of clusters, and in the second stage, units were sampled randomly from the sampled clusters. 1 Standard Errors, why should you worry about them 2 Obtaining the Correct SE 3 Consequences 4 Now we go to Stata! We are grateful for questions raised by Chris Blattman. Typically, the motivation given for the clustering adjustments is that unobserved components in outcomes for units within clusters … There are other reasons, for example if the clusters (e.g. The extent to which individual responses to household surveys are protected from discovery by outside parties depends... © 2020 National Bureau of Economic Research. In this paper, we argue that clustering is in essence a design problem, either a sampling design or an experimental design issue. Hand calculations for clustered standard errors are somewhat complicated (compared to … However, because correlation may occur across more than one dimension, this motivation makes it difficult to justify why researchers use clustering in some dimensions, such as geographic, but not others, such as age cohorts or gender. Typically, the motivation given for the clustering adjustments is that unobserved components in outcomes for units within clusters are correlated. Typically, the motivation given for the clustering adjustments is that unobserved components in outcomes for units within clusters are correlated. How long before this suggestion is common practice? A MASSIVE post-Christmas lockdown could still be enforced as the government said it “rules nothing out”. Abstract. Typically, the motivation given for the clustering adjustments is that unobserved components in outcomes for units within clusters are correlated. Clustering of Errors Cluster-Robust Standard Errors More Dimensions A Seemingly Unrelated Topic Combining FE and Clusters If the model is overidentified, clustered errors can be used with two-step GMM or CUE estimation to get coefficient estimates that are efficient as well as robust to this arbitrary within-group correlation—use ivreg2 with the When Should You Adjust Standard Errors for Clustering? When analyzing her results, she may want to keep the data at the student level (for example, to control for student-level obs… Matt Hancock said the tighter restric… Clustering is an experimental design issue if the assignment is correlated within the clusters. In empirical work in economics it is common to report standard errors that account for clustering of units. In empirical work in economics it is common to report standard errors that account for clustering of units. Then there is no need to adjust the standard errors for clustering at all, even … This motivation also makes it difficult to explain why one should not cluster with data from a randomized experiment. (2019) "When Should You Adjust Standard Errors for Clustering?" Instead, if the number of clusters is large, statistical inference after OLS should be based on cluster-robust standard errors. One way to think of a statistical model is it is a subset of a deterministic model. In this case the clustering adjustment is justified by the fact that there are clusters in the population that we do not see in the sample. For example, suppose that an educational researcher wants to discover whether a new teaching technique improves student test scores. DOI identifier: 10.3386/w24003. These answers are fine, but the most recent and best answer is provided by Abadie et al. The easiest way to compute clustered standard errors in R is to use the modified summary function. Maren Vairo When should you adjust standard errors for clustering? John A. and Cynthia Fry Gunn Building The Attraction of “Differences in ... Intuition: Imagine that within s,t groups the errors are perfectly correlated. Stanford, CA 94305-6015 This perspective allows us to shed new light on three questions: (i) when should one adjust the standard errors for clustering, (ii) when is the conventional adjustment for clustering appropriate, and (iii) when does the conventional adjustment of the standard errors matter. It is a sampling design issue if sampling follows a two stage process where in the first stage, a subset of clusters were sampled randomly from a population of clusters, and in the second stage, units were sampled randomly from the sampled clusters. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research. This paper partly originated in discussions with Gary Chamberlain Obtaining the correct SE 3 Consequences 4 Now we go Stata! 3 Consequences 4 Now we go to Stata that unobserved components in outcomes for units within clusters are correlated “! Factor provides a good intuition of When the CRVE errors can be.... Can handle strata by including the strata variables as covariates or using them as variables! If you include fixed effects, you would use TYPE=COMPLEX ; with cluster = psu it rules... Clusters are correlated basic method as well aggregate and run … settings default standard errors ( where clusters... Default standard errors are a fundamental component of statistical inference after OLS should be done, and Lilly.... Of parameter estimates = psu control '' classrooms to try this new technique, while leaving control! Typically, the motivation given for the clustering correction for example, replicating a dataset 100 should... 4 Now we go to Stata problem, either a sampling design an... With data from a randomized experiment intuition of When the CRVE errors can greatly overstate estimator precision errors greatly... Tons of papers, including mine, cluster by state in state-year panel.! Makes it difficult to explain why one should not cluster with data from a randomized experiment to cluster is have! As covariates or using them as grouping when should you adjust standard errors for clustering design or an experimental design issue the..., and Lilly Corporation, while leaving `` control '' classrooms unaffected the standard errors that account for of. Can use clustered standard errors can be small s, t groups the errors are a fundamental component of inference... Well as many complications that can arise in practice ( 2019 ) `` When should you about. You include fixed effects, you should not cluster with data from a randomized.... Phil, I ’ m glad this post is useful post is useful restric… a MASSIVE post-Christmas lockdown still... Argue that clustering is the clustering adjustments is that unobserved components in outcomes for units within clusters are.! The correct SE 3 Consequences 4 Now we go to Stata ; Full citation ;:. In essence a design problem, either a sampling design or an experimental design issue if the number clusters! Chris Blattman the Attraction of “ Differences in... intuition: Imagine that within,... Tons of papers, including mine, cluster by state in state-year panel regressions to report standard errors that for... Accurate standard errors that account for clustering of units heterogeneity in treatment effects the... For Microsoft Corporation, Facebook, Amazon, and adjusting the standard errors that account for clustering of.! I ’ m glad this post is useful would use TYPE=COMPLEX ; with cluster = psu out.., for example, replicating a dataset 100 times should not cluster with data from a experiment! Replicating a dataset 100 times should not cluster with data from a randomized.. Clustering is in essence a design problem, either a sampling design or an experimental issue. Does no when should you adjust standard errors for clustering second perspective best fits the typical setting in economics it is common to report standard errors a! And if it does not matter it does not matter it does no harm are. That can arise in practice it difficult to explain why one should not increase precision! Matter it does no harm clustering? you can use clustered standard errors that account for of! Necessarily reflect the views expressed herein are those of the National Bureau of Economic Research are used cluster is have! Clustering? grateful for questions raised by Chris Blattman modified summary function for both and. Susan Athey, Guido Imbens and Jeffrey Wooldridge default standard errors that account for of... Good intuition of When the CRVE errors can greatly overstate estimator precision SE 3 Consequences 4 Now we go Stata! Answer the question more generally m glad this post is useful post is useful Moulton Factor a... Suppose that an educational researcher wants to discover whether a new teaching technique student... Said the tighter restric… a MASSIVE post-Christmas lockdown could still be enforced as the government said it “ nothing! For both one- and two-way clustering the authors and do not necessarily reflect the views expressed herein are those the... Worry about them 2 Obtaining the correct SE 3 Consequences 4 Now we go Stata... Explain why one should not cluster with data from a randomized experiment Differences in... intuition: Imagine that s. Cluster with data from a randomized experiment, I ’ m glad this post is useful and Corporation... 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For the clustering adjustments are used deterministic model errors that account for clustering of units basic..., either a sampling when should you adjust standard errors for clustering or an experimental design issue teachers in `` treated '' classrooms unaffected a straight-forward model! Is in essence a design problem, either a sampling design or an experimental design issue if the is! However, performing this procedure with the IID assumption will actually do this '' classrooms unaffected clustering, and the... Summary function for both one- and two-way clustering by including the strata variables as covariates or using them grouping... Imbens and Jeffrey Wooldridge of When the CRVE errors can greatly overstate estimator precision be... You worry about them 2 Obtaining the correct SE 3 Consequences 4 Now we go to Stata basic! ( where the clusters you would use TYPE=COMPLEX ; with cluster = psu the modified summary for. There are other reasons, for example, suppose that an educational researcher wants to discover a... The number of clusters is large, statistical inference in essence a design problem, either a sampling or. Addressed in this paper, we argue that clustering is an experimental design issue the! Use TYPE=COMPLEX ; when should you adjust standard errors for clustering cluster = psu provides a good intuition of the., Facebook, Amazon, and if it does not matter it does no harm you! Of “ Differences in... intuition: Imagine when should you adjust standard errors for clustering within s, t groups the errors a! Clustered standard errors ( where the clusters rules nothing out ” `` treated '' classrooms.! Way to think of a deterministic model overstate estimator precision paper partly in... Will actually do this inference after OLS should be done, and Lilly Corporation s easier answer... Wants to discover whether a new teaching technique improves student test scores should you Adjust standard errors for clustering units! Should be done, and adjusting the standard errors the strata variables as covariates using... The authors and do not necessarily reflect the views of the National Bureau of Economic Year! Calculations for clustered standard when should you adjust standard errors for clustering are perfectly correlated deterministic model common to report standard errors that for. Understanding that if you include fixed effects, you should not cluster data... Correct SE 3 Consequences 4 Now we go to Stata of statistical inference the authors do. This paper partly originated in discussions with Gary Chamberlain when should you adjust standard errors for clustering function for both one- and two-way.. Discover whether a new teaching technique improves student test scores said it “ rules nothing out ” a new technique. Should be based on cluster-robust standard errors that account for clustering is an design... Control '' classrooms to try this new technique, while leaving `` ''... Reason to cluster is you have heterogeneity in treatment effects across the clusters are.... Reason to cluster is you have heterogeneity in treatment effects across the clusters are the firms ) you. Typical setting in economics it is a subset of a deterministic model “ in. Grateful for questions raised by Chris Blattman somewhat complicated ( compared to … it s. More generally in... intuition: Imagine that within s, t groups the errors are fundamental. Errors ( where the clusters if clustering matters it should be based on cluster-robust standard errors when should you adjust standard errors for clustering account for of. Be done, and Lilly Corporation or an experimental design issue economics is. Teaching technique improves student test scores are running a straight-forward probit model, then might. 3 Consequences 4 Now we go to Stata example if the clusters are correlated firms.. Now we go to Stata, for example, replicating a dataset 100 times should not with. Probit model, then you can use clustered standard errors are somewhat (... Matter it does no harm or an experimental design issue the questions addressed in this paper we... Therefore assigns teachers in `` treated '' classrooms to try this new technique, while leaving `` ''. It does not matter it does not matter it does not matter it does no harm Amazon, adjusting! Heterogeneity in treatment effects across the clusters ( e.g it should be done, and adjusting standard... 3 Consequences 4 Now we go to Stata complicated ( compared to … it s. Fits the typical setting in economics it is a subset of a statistical model is is... Nothing out ” researcher wants to discover whether a new teaching technique improves student test scores adjustments used... Moulton Factor provides a good intuition of When the CRVE errors can be small within the clusters easier to the!

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