Trade-weighted aggregated tariffs (TWPT) are often used in analyzing the issues of

erosion of non-reciprocal preferences. This paper argues that commonly used TWPTs

under-estimate the true protection on imports originated from preference-receiving

countries, including LDCs. When used in numerical simulations of preference erosion

and expansion scenarios, the TWPTs tend to incorrectly downplay preference erosion

effect of MFN tariff cuts, and understate the export promotion effect of expanding

preferences. In light of these claims, an alternative aggregation scheme is developed

to calculate aggregated preferential tariffs imposed by a number of developed countries

on African LDCs. These are shown to be higher than the TWPTs aggregated

from the same disaggregated tariff data set. Numerical simulations conducted with

the two sets of aggregated tariffs confirm the two claims and suggest that TWPTs may

lead to misleading policy implications concerning expanding preferences for the

LDCs.

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