formula.fixest: Extract the formula of a 'fixest' fit

In fixest: Fast Fixed-Effects Estimations

Extract the formula of a fixest fit

Description

This function extracts the formula from a fixest estimation (obtained with femlm, feols or feglm). If the estimation was done with fixed-effects, they are added in the formula after a pipe (“|”). If the estimation was done with a non linear in parameters part, then this will be added in the formula in between I().

Usage

## S3 method for class 'fixest'
formula(x, type = "full", fml.update = NULL, fml.build = NULL, ...)

## S3 method for class 'fixest_multi'
formula(x, type = "full", fml.update = NULL, fml.build = NULL, ...)

Arguments

x	An object of class fixest. Typically the result of a femlm, feols or feglm estimation.
type	A character scalar. Default is type = "full" which gives back a formula containing the linear part of the model along with the fixed-effects (if any) and the IV part (if any). Here is a description of the other types: full.noiv: the full formula without the IV part full.nofixef.noiv: the full formula without the IV nor the fixed-effects part lhs: a one-sided formula with the dependent variable rhs: a one-sided formula of the right hand side without the IVs (if any) rhs.nofixef or indep: a one-sided formula of the right hand side without the fixed-effects nor IVs (if any), it is equivalent to the independent variables NL: a one-sided formula with the non-linear part (if any) fixef: a one-sided formula containing the fixed-effects iv: a two-sided formula containing the endogenous variables (left) and the instruments (right) iv.endo: a one-sided formula of the endogenous variables iv.inst: a one-sided formula of the instruments iv.reduced: a two-sided formula representing the reduced form, that is y ~ exo + inst
fml.update	A formula representing the changes to be made to the original formula. By default it is NULL. Use a dot to refer to the previous variables in the current part. For example: . ~ . + xnew will add the variable xnew as an explanatory variable. Note that the previous fixed-effects (FEs) and IVs are implicitly forwarded. To rerun without the FEs or the IVs, you need to set them to 0 in their respective slot. Ex, assume the original formula is: y ~ x | fe | endo ~ inst, passing . ~ . + xnew to fml.update leads to y ~ x + xnew | fe | endo ~ inst (FEs and IVs are forwarded). To add xnew and remove the IV part: use . ~ . + xnew | . | 0 which leads to y ~ x + xnew | fe.
fml.build	A formula or NULL (default). You can create a new formula based on the parts of the formula of the object in x. In this argument you have access to these specific variables: .: to refer to the part of the original formula .lhs: to refer to the dependent variable .indep: to refer to the independent variables (excluding the fixed-effects) .fixef: to refer to the fixed-effects .endo: to refer to endogenous variables in an IV estimation .inst: to refer to instruments in an IV estimation Example, the original estimation was y ~ x1 | z ~ inst. Then fml.build = . ~ .endo + . leads to y ~ z + x1.
...	Not currently used.

Details

The arguments type, fml.update and fml.build are exclusive: they cannot be used at the same time.

Value

It returns either a one-sided formula, either a two-sided formula.

Author(s)

Laurent Berge

See Also

See also the main estimation functions femlm, feols or feglm. model.matrix.fixest, update.fixest, summary.fixest, vcov.fixest.

Examples

# example estimation with IVS and FEs
base = setNames(iris, c("y", "x1", "endo", "instr", "species"))
est = feols(y ~ x1 | species | endo ~ instr, base)

# the full formula
formula(est)

# idem without the IVs nor the FEs
formula(est, "full.nofixef.noiv")

# the reduced form
formula(est, "iv.reduced")

# the IV relation only
formula(est, "iv")

# the dependent variable => onse-sided formula
formula(est, "lhs")

# using update, we add x1^2 as an independent variable:
formula(est, fml.update = . ~ . + x1^2)

# using build, see the difference => the FEs and the IVs are not inherited
formula(est, fml.build = . ~ . + x1^2)

# we can use some special variables
formula(est, fml.build = . ~ .endo + .indep)

fixest documentation built on March 18, 2026, 9:06 a.m.