I really need help on my panel regression. My dependent variable is bilateral trade flow. My Independent variables are GDP, Exchange Rate, Inflation, and Transportation cost (as a proxy variable to distance, since distance is constant so numbers are fixed and can make my results worst). Gravity model states that GDP and Distance must be negative. I log my variables, I tried experimenting and combing logs to constant variables. I did make Transportation cost turn to negative, my only problem is that the GDP holds its positive sign in anything that I do. Please help, I really need to finish this because I want to graduate. Thanks to anyone who is/are willing to help.