I am going to run gravity model for my thesis. My purpose is to find the effect of lpi on export and import so I choose export and import as dep var and GDP, distance, lpi, RTA and landlocked as indep var . In fact in my collected data, there are so many cases in which export , import value equals to 0 ( 245 cases for export and 269 for import)
Code:
 sum export import gdp distance landlocked RTA lpi0 lpi1 lpi2 lpi3 lpi4 lpi5 lpi6

    Variable |        Obs        Mean    Std. Dev.       Min        Max
-------------+---------------------------------------------------------
      export |      1,336    828.4553    3016.076          0   41549.71
      import |      1,336    832.2992    3950.458          0   58532.57
         gdp |      1,336    4506.944    16476.49    1.97454   196236.7
    distance |      1,336    8808.857    4342.772   216.4326   19228.99
  landlocked |      1,336    .2095808    .4071619          0          1
-------------+---------------------------------------------------------
         RTA |      1,336    .0905689    .2871025          0          1
        lpi0 |      1,254    2.881038    .5772499   1.598322   4.225967
        lpi1 |      1,254    2.693316    .5977922   1.111111    4.20779
        lpi2 |      1,254    2.756885    .6817719   1.237654   4.439356
        lpi3 |      1,254    2.848861    .5226796   1.362654      4.235
-------------+---------------------------------------------------------
        lpi4 |      1,254    2.830826    .6080419   1.394253    4.31065
        lpi5 |      1,254    2.886995     .629167   1.513605   4.377678
        lpi6 |      1,254    3.256001    .5821565   1.665079   4.795714
I have some matters as below . Please help me
1. In my data, the cases in which export and import value is 0 account for 18% of total observation . So is this necessary for me to use other regression not OLS as OLS can make bias estimation ?
2. As I saw in some theses, the authors used PPML,OLS truncated and Heckman with high of zero trade bilateral . I have tried with PPML and run my data as following as :
Code:
ppml export ldis lgdp llpi0 landlocked RTA, cluster(country1)
Can you instruct me how to run by OLS truncated and Heckman?
Thank so much