## What is the difference between clustering and fixed effects?

Cluster-adjusted standard error take into account standard error but leave your point estimates unchanged (standard error will usually go up)! Fixed-effects estimation takes into account unobserved time-invariant heterogeneity (as you mentioned).

## Can I use fixed effects and clustered standard errors?

It is perfectly acceptable to use fixed effects and clustered errors at the same time or independently from each other.

**Should you cluster standard errors?**

The general rule is that you still need to cluster if either the sampling or assignment to treatment was clustered. However, the authors show that cluster adjustments will only make an adjustment with fixed effects if there is heterogeneity in treatment effects.

**When should you not cluster standard errors?**

state in their conclusion: if the sampling process is not clustered and the treatment assignment is not clustered, you should not cluster standard errors even if clustering changes your standard errors. Clustering will yield approximately correct standard errors in the following three possible cases.

### At what level should you cluster standard errors?

Instead, we show that researchers should cluster their standard errors at the pair level. Using simulations, we show that those results extend to stratified experiments with few units per strata.

### What happens when you cluster standard errors?

Clustered Standard Errors(CSEs) happen when some observations in a data set are related to each other. This correlation occurs when an individual trait, like ability or socioeconomic background, is identical or similar for groups of observations within clusters.

**Why should you adjust standard errors for clustering?**

Typically, the motivation given for the clustering adjustments is that unobserved components in outcomes for units within clusters are correlated. 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.

**How many clusters is too few?**

There is no clear-cut definition of “few”; depending on the situation “few” may range from less than 20 to less than 50 clusters in the balanced case. We focus on OLS, for simplicity and because this is the most commonly-used estimation method in practice.

#### Why clustered standard errors are higher?

In such DiD examples with panel data, the cluster-robust standard errors can be much larger than the default because both the regressor of interest and the errors are highly correlated within cluster. This serial correlation leads to a potentially large difference between cluster-robust and default standard errors.

#### How do you cluster standard error?

One way to control for Clustered Standard Errors is to specify a model. For example, you could specify a random coefficient model or a hierarchical model. However, accuracy of any calculated SEs completely relies upon you specifying the correct model for within-cluster error correlation.

**How many clusters do you need for clustered standard errors?**

While no specific number of clusters is statistically proven to be sufficient, practitioners often cite a number in the range of 30-50 and are comfortable using clustered standard errors when the number of clusters exceeds that threshold.

**Can clustered standard errors be smaller?**

cluster-robust standard errors are smaller than unclustered ones in fgls with cluster fixed effects.

## When to use fixed effects vs clustered standard errors?

In these cases, it is usually a good idea to use a fixed-effects model. Clustered standard errors are for accounting for situations where observations WITHIN each group are not i.i.d. (independently and identically distributed). A classic example is if you have many observations for a panel of firms across time.

## When do you use clustered standard errors in regression?

When there is both heteroskedasticity and autocorrelation so-called heteroskedasticity and autocorrelation-consistent (HAC) standard errors need to be used. Clustered standard errors belong to these type of standard errors.

**How are standard errors in fixed effects models different?**

We also briefly discuss standard errors in fixed effects models which differ from standard errors in multiple regression as the regression error can exhibit serial correlation in panel models. In the fixed effects model Y it = β1Xit +αi +uit , i = 1,…,n, t = 1,…,T, Y i t = β 1 X i t + α i + u i t , i = 1, …, n, t = 1, …, T, we assume the following:

**Do you use group fixed effect or standard error?**

Both approaches, using group fixed effects and/or cluster-adjusted standard error take into account different issues related to clustered (or panel) data and I would clearly view them as distinct approaches. Often you want to use both of them: