R plm lag - what is the equivalent to L1.x in Stata? - r

Question

Using the plm package in R to fit a fixed-effects model, what is the correct syntax to add a lagged variable to the model? Similar to the 'L1.variable' command in Stata.
Here is my attempt adding a lagged variable (this is a test model and it might not make sense):
library(foreign)
nlswork <- read.dta("http://www.stata-press.com/data/r11/nlswork.dta")
pnlswork <- plm.data(nlswork, c('idcode', 'year'))
ffe <- plm(ln_wage ~ ttl_exp+lag(wks_work,1)
, model = 'within'
, data = nlswork)
summary(ffe)
R output:
Oneway (individual) effect Within Model
Call:
plm(formula = ln_wage ~ ttl_exp + lag(wks_work), data = nlswork,
model = "within")
Unbalanced Panel: n=3911, T=1-14, N=19619
Residuals :
Min. 1st Qu. Median 3rd Qu. Max.
-1.77000 -0.10100 0.00293 0.11000 2.90000
Coefficients :
Estimate Std. Error t-value Pr(>|t|)
ttl_exp 0.02341057 0.00073832 31.7078 < 2.2e-16 ***
lag(wks_work) 0.00081576 0.00010628 7.6755 1.744e-14 ***
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
Total Sum of Squares: 1296.9
Residual Sum of Squares: 1126.9
R-Squared: 0.13105
Adj. R-Squared: -0.085379
F-statistic: 1184.39 on 2 and 15706 DF, p-value: < 2.22e-16
However, I got different results compared what Stata produces.
In my actual model, I would like to instrument an endogenous variable with its lagged value.
Thanks!
For reference, here is the Stata code:
webuse nlswork.dta
xtset idcode year
xtreg ln_wage ttl_exp L1.wks_work, fe
Stata output:
Fixed-effects (within) regression Number of obs = 10,680
Group variable: idcode Number of groups = 3,671
R-sq: Obs per group:
within = 0.1492 min = 1
between = 0.2063 avg = 2.9
overall = 0.1483 max = 8
F(2,7007) = 614.60
corr(u_i, Xb) = 0.1329 Prob > F = 0.0000
------------------------------------------------------------------------------
ln_wage | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
ttl_exp | .0192578 .0012233 15.74 0.000 .0168597 .0216558
|
wks_work |
L1. | .0015891 .0001957 8.12 0.000 .0012054 .0019728
|
_cons | 1.502879 .0075431 199.24 0.000 1.488092 1.517666
-------------+----------------------------------------------------------------
sigma_u | .40678942
sigma_e | .28124886
rho | .67658275 (fraction of variance due to u_i)
------------------------------------------------------------------------------
F test that all u_i=0: F(3670, 7007) = 4.71 Prob > F = 0.0000

Answer 1

lag() as it is in plm lags the observations row-wise without "looking" at the time variable, i.e. it shifts the variable (per individual). If there are gaps in the time dimension, you probably want to take the value of the time variable into account. There is the (as of now) unexported function plm:::lagt.pseries which takes the time variable into account and hence handles gaps in data as you might expect.
Edit: Since plm version 1.7-0, default behaviour of lag in plm is to shift time-wise but one can control behaviour by argument shift(shift = c("time", "row")) to shift either time-wise or row-wise (old behaviour).
Use it as follows:
library(plm)
library(foreign)
nlswork <- read.dta("http://www.stata-press.com/data/r11/nlswork.dta")
pnlswork <- pdata.frame(nlswork, c('idcode', 'year'))
ffe <- plm(ln_wage ~ ttl_exp + plm:::lagt.pseries(wks_work,1)
, model = 'within'
, data = pnlswork)
summary(ffe)
Oneway (individual) effect Within Model
Call:
plm(formula = ln_wage ~ ttl_exp + plm:::lagt.pseries(wks_work,
1), data = nlswork, model = "within")
Unbalanced Panel: n=3671, T=1-8, N=10680
Residuals :
Min. 1st Qu. Median 3rd Qu. Max.
-1.5900 -0.0859 0.0000 0.0957 2.5600
Coefficients :
Estimate Std. Error t-value Pr(>|t|)
ttl_exp 0.01925775 0.00122330 15.7425 < 2.2e-16 ***
plm:::lagt.pseries(wks_work, 1) 0.00158907 0.00019573 8.1186 5.525e-16 ***
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
Total Sum of Squares: 651.49
Residual Sum of Squares: 554.26
R-Squared: 0.14924
Adj. R-Squared: -0.29659
F-statistic: 614.604 on 2 and 7007 DF, p-value: < 2.22e-16
Btw1: Better use pdata.frame() instead of plm.data().
Btw2: You can check for gaps in your data with plm's is.pconsecutive():
is.pconsecutive(pnlswork)
all(is.pconsecutive(pnlswork))
You can also make the data consecutive first and then use lag(), like this:
pnlswork2 <- make.pconsecutive(pnlswork)
pnlswork2$wks_work_lag <- lag(pnlswork2$wks_work)
ffe2 <- plm(ln_wage ~ ttl_exp + wks_work_lag
, model = 'within'
, data = pnlswork2)
summary(ffe2)
Oneway (individual) effect Within Model
Call:
plm(formula = ln_wage ~ ttl_exp + wks_work_lag, data = pnlswork2,
model = "within")
Unbalanced Panel: n=3671, T=1-8, N=10680
Residuals :
Min. 1st Qu. Median 3rd Qu. Max.
-1.5900 -0.0859 0.0000 0.0957 2.5600
Coefficients :
Estimate Std. Error t-value Pr(>|t|)
ttl_exp 0.01925775 0.00122330 15.7425 < 2.2e-16 ***
wks_work_lag 0.00158907 0.00019573 8.1186 5.525e-16 ***
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
Total Sum of Squares: 651.49
Residual Sum of Squares: 554.26
R-Squared: 0.14924
Adj. R-Squared: -0.29659
F-statistic: 614.604 on 2 and 7007 DF, p-value: < 2.22e-16
Or simply:
ffe3 <- plm(ln_wage ~ ttl_exp + lag(wks_work)
, model = 'within'
, data = pnlswork2) # note: it is the consecutive panel data set here
summary(ffe3)
Oneway (individual) effect Within Model
Call:
plm(formula = ln_wage ~ ttl_exp + lag(wks_work), data = pnlswork2,
model = "within")
Unbalanced Panel: n=3671, T=1-8, N=10680
Residuals :
Min. 1st Qu. Median 3rd Qu. Max.
-1.5900 -0.0859 0.0000 0.0957 2.5600
Coefficients :
Estimate Std. Error t-value Pr(>|t|)
ttl_exp 0.01925775 0.00122330 15.7425 < 2.2e-16 ***
lag(wks_work) 0.00158907 0.00019573 8.1186 5.525e-16 ***
---
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
Total Sum of Squares: 651.49
Residual Sum of Squares: 554.26
R-Squared: 0.14924
Adj. R-Squared: -0.29659
F-statistic: 614.604 on 2 and 7007 DF, p-value: < 2.22e-16