不明觉厉，最好详细点，加分

有个地理距离的说明，整理了一下
Country-level variables
• iso2, iso3, cnum: ISO codes in two and three characters, and in three numbers respectively. 3
• country, pays: Name of country in English and French respectively.4
• area: Country’s area in km2.
dis_int: Internal distance of country i, dii = .67parea/π (an often used measure of
average distance between producers and consumers in a country, see Head and Mayer, 2002
for more on this topic).
• landlocked: Dummy variable set equal to 1 for landlocked countries.
• continent: Continent to which the country is belonging
• langoff_i: Official or national languages and languages spoken by at least 20% of the
population of the country (and spoken in another country of the world5) following the same
logic than the “open-circuit languages” in Mélitz (2002).
• lang20_i: Languages (mother tongue, lingua francas or second languages) spoken by at
least 20% of the population of the country.
• lang9_i: Languages (mother tongue, lingua francas or second languages) spoken by between 9% and 20% of the population of the country. 6.
• colonizeri: Colonizers of the country for a relatively long period of time and with a
substantial participation in the governance of the colonized country.
• short_colonizeri: Colonizers of the country for a relatively short period of time or
with only low involvement in the governance of the colonized country
• city_en, city_fr: Names of capitals or main cities of the country in English and
French.
• lat, lon: Latitude and longitude of the city.9
• cap: Variable equals to 1 if the city is the capital of the country, to 0 if the city is the most
populated city (maincity equals to 1) but not the capital, and to 2 in the cases of two
capitals, if the city is the most populated but the “second” capital or the previous capital10.
• maincity: Variable coded as 1 when the city is the most populated of the country and as
2 otherwise11.
• citynum: Number of cities for each country used to calculate our weighted distances described in the next section.
Finally the dist_cepii.xls file provides also dummy variables indicating whether the two
countries are contiguous (contig), share a common language, have had a common colonizer
after 1945 (comcol), have ever had a colonial link (colony), have had a colonial relationship
after 1945 (col45), are currently in a colonial relationship (curcol)14 or were/are the same
country (smctry)15.