Rural Interdisciplinary Sociology of Community Seed-Bank Custodian Term Limits and Farmer Variety Loss in Zimbabwean Ward Assemblies: A Regression Discontinuity Analysis of Tenure Thresholds

Community seed banks in Zimbabwean ward assemblies rotate custodians at fixed tenure thresholds, a governance rule intended to prevent elite capture but one whose consequences for farmer variety loss remain unexamined. Because no observational data are available, the study characterises the operating characteristics of the proposed design under a declared variance structure: between-cluster standard deviation of 6.0, residual standard deviation of 4.5, and intraclass correlation of 0.64. Power was estimated across 2000 replications per design cell for cluster counts of 8, 16, 24 and 32 and effect sizes of 3, 5 and 8 percentage points. The analysis indicates that 52 clusters are required to detect a 5-percentage-point effect at 80% power, a requirement that exceeds the largest design cell examined. Minimum detectable effects at 80% power remain above 5 percentage points for all cluster counts considered. The tenure threshold generates a discontinuity that is identifiable in principle but statistically fragile in the ward-assembly setting, and it specifies the conditions under which a credible causal estimate of custodian rotation on variety loss could be recovered.