The function fits first difference models using GLS estimation.

## Arguments

- formula
Model formula. See details for crucial info on

`panelr`

's formula syntax.- data
The data, either a

`panel_data`

object or`data.frame`

.- id
If

`data`

is not a`panel_data`

object, then the name of the individual id column as a string. Otherwise, leave as NULL, the default.- wave
If

`data`

is not a`panel_data`

object, then the name of the panel wave column as a string. Otherwise, leave as NULL, the default.- use.wave
Should the wave be included as a predictor? Default is FALSE.

- min.waves
What is the minimum number of waves an individual must have participated in to be included in the analysis? Default is

`2`

and any valid number is accepted.`"all"`

is also acceptable if you want to include only complete panelists.- variance
One of

`"toeplitz-1"`

,`"constrained"`

, or`"unconstrained"`

. The toeplitz variance specification estimates a single error variance and a single lag-1 error correlation with other lags having zero correlation. The constrained model assumes no autocorrelated errors or heteroskedastic errors. The unconstrained option allows separate variances for every period as well as every lag of autocorrelation. This can be very computationally taxing as periods increase and will be inefficient when not necessary. See Allison (2019) for more.- error.type
Either "CR2" or "CR1S". See the

`clubSandwich`

package for more details.- ...
Ignored.

## References

Allison, P. D. (2019). Asymmetric fixed-effects models for panel data.
*Socius*, *5*, 1-12. https://doi.org/10.1177/2378023119826441

## Examples

```
data("teen_poverty")
# Convert to long format
teen <- long_panel(teen_poverty, begin = 1, end = 5)
model <- fdm(hours ~ lag(pov) + spouse, data = teen)
#> Loading required namespace: clubSandwich
#> Registered S3 method overwritten by 'clubSandwich':
#> method from
#> bread.mlm sandwich
summary(model)
#> MODEL INFO:
#> Entities: 1151
#> Time periods: 3-5
#> Dependent variable: hours
#> Variance structure: toeplitz-1 (theta = -0.51)
#>
#> MODEL FIT:
#> AIC = 28771.97, BIC = 28802.7
#>
#> Standard errors: CR2
#> ------------------------------------------------
#> Est. S.E. t val. p
#> ----------------- ------- ------ -------- ------
#> (Intercept) 2.09 0.19 10.89 0.00
#> lag_pov -1.62 0.56 -2.91 0.00
#> spouse -2.38 1.26 -1.89 0.06
#> ------------------------------------------------
```