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106 Kochi University Review, No. 97area. Frequency of use of varieties by respondents, identification of oldest and newestvarieties planted, and share to total area planted by variety were clarified u....

106 Kochi University Review, No. 97area. Frequency of use of varieties by respondents, identification of oldest and newestvarieties planted, and share to total area planted by variety were clarified using thesecross-section data sets.2) Analysis of adoption and spatial diversityTo better understand the dynamics of rice variety adoption, two indices such asthe proportion of recent varieties and weighted average age of varieties were calculatedto determine the adoption rate of the newly released varieties following Brennan (1984).The proportion of recent varieties is estimated as the proportion of the total areathat is planted with recently released varieties. This index, qit, is computed as follows:qit = pit if year of release ? t-m, while qit = 0 if year of release < t-m, where pit is theproportion of the area sown with variety, i, in year, t; m is the number of years used todefine “recent”. TheniIt = Σqitwhere I is the proportion of the total area that is sown with varieties released in theprevious m years. In this study, we assumed a lag of two years between the release ofa variety and its availability to farmers, and defined ‘recent’ variety as a variety that isavailable to farmers for five more years, thus in this study we used m=7.The weighted average age of varieties3, on the other hand, consists of the weightedaverage age of varieties grown by farmers in a given year, measured in years fromvarietal release and weighted by the proportion of area sown with each variety at thattime. This index, WAt, is computed for a given year, t, as follows:WAt = pit Riti Σwhere Rit is the ageof the variety in terms of the number of years (at time t) since the release of variety i.This measure avoids the use of an arbitrary definition of “new” or “recent” varieties(Brennan et al, 1991).Varietal diversity is presented in this study by determining the number ofreleased varieties planted by farmers in a province in a given year and season, andby determining the variety groups planted by farmers (Estudillo et al., 2006). Theestimated proportion of area planted with only one or a few varieties in a province wasalso described to show the diversity in varieties planted.