This proxy-variable is given by the number of months in a year in which transactions of this firm-good-country combination are recorded. Note that within a month all exports or imports of a specific HS6-good to a specific
country by a firm are aggregated and reported as one data point only.
Therefore,
the proxy for trade frequency used here may be biased for high frequency traders
which trade the same good with the same country in (nearly) every month several
times. For low frequency traders, however, the number of months with recorded
transactions is a reliable approximation (see the discussion in Békés et al. 2015).