However, even similar surveys may not be strictly comparable
because of differences in timing, sampling frames, or the quality
and training of enumerators. Comparisons of countries at different
levels of development also pose a potential problem because of differences
in the relative importance of the consumption of nonmarket
goods. The local market value of all consumption in kind (including
own production, particularly important in underdeveloped rural economies)
should be included in total consumption expenditure, but
may not be. Most survey data now include valuations for consumption
or income from own production, but valuation methods vary.