XS = is export supply measured in tons
RPP = the ratio of the producer price to the domestic price index
RPX = the ratio of the export price to the producer price
XC = agricultural export credit
RNF = average annual rainfall in millimetres
RD = classified road network
DTD = dummy variable for deregulation of both domestic and export marketing
activities, which takes the value of 0 for years prior to deregulation and 1
for years thereafter
DPF = dummy variable for abandonment of producer price fixing, which takes
the value of 0 for period of price fixing, and 1 afterwards
DICA = dummy variable for ICA quotas that takes the value of 0 for years when
quotas were not in force and 1 for years of ICA quotas