For estimating the equation (1), we construct panel data for the period between 1980
and 2010 with 174 countries as Thailand’s trading partners identified in the statistics of
International Monetary Fund (IMF).To avoid the problem of sample selection bias in our
panel data which contain a large number of missing data in a specific group of countries
such as small-size countries, we adopt the truncated regression model with dependent
variables left-censored at zero and with the normal distribution for the error term