Hello
My data is panel data with strongly balanced . As I want to know the effect of lpi on export and import I have chosen export and import as dep vars and lpi, gdp, distance and dummy as indep vars. The summaries of my data as follow as
Code:
    Variable |        Obs        Mean    Std. Dev.       Min        Max
-------------+---------------------------------------------------------
      export |      1,328     833.446    3024.465          0   41549.71
      import |      1,328     837.313    3961.817          0   58532.57
    distance |      1,328    8860.618    4304.137    478.553   19228.99
         gdp |      1,328     4523.65    16524.65    1.97454   196236.7
  landlocked |      1,328    .2108434    .4080611          0          1
-------------+---------------------------------------------------------
        lpi0 |      1,262    2.879012    .5787635   1.598322   4.225967
        lpi1 |      1,262    2.691696    .5987479   1.111111    4.20779
        lpi2 |      1,262    2.754088    .6829058   1.237654   4.439356
        lpi3 |      1,262    2.846396    .5248384   1.362654      4.235
        lpi4 |      1,262    2.828908     .608916   1.394253    4.31065
-------------+---------------------------------------------------------
        lpi5 |      1,262    2.886015    .6297591   1.513605   4.377678
        lpi6 |      1,262    3.253649    .5854234   1.665079   4.795714
Because the cases with the export or import value is equal to 0 account for about 18% of total obs so I decided to use pplm to analyse. My equation becomes like this
Ex= a ln(gdp) + b ln(dis) + c ln( lpi) + e. dummy ( landlocked)
But there are some missing data on lpi because in some years in some specific countries , LPI were not collected
Code:
 
gen ll1=ln(lpi1)
(66 missing values generated)
So I wonder whether my equation is suitable or not. If not what is equation should I use?
Please give me advice
Thank so much