Rural cooperative warehouse receipt collateral thresholds and smallholder post-harvest storage duration in Tanzanian wards: a regression-discontinuity estimation of collateral-eligibility cutoffs

Warehouse receipt systems in Tanzanian rural cooperatives typically extend collateral eligibility only to members whose stored volume exceeds a fixed threshold, creating a sharp discontinuity in access to post-harvest credit. Because no observational data are available for the present analysis, the study characterises the operating characteristics of a regression-discontinuity design applied to this threshold rather than estimating the effect itself. A replication-based evaluation was conducted across a design grid of 8, 16, 24 and 32 clusters, effect sizes of 3, 5 and 8 percentage points, and 2,000 replications per cell, under a declared variance structure with between-cluster SD = 6.0, residual SD = 4.5 and intraclass correlation = 0.64. Power was estimated for the before-after-control-impact interaction, and the minimum detectable effect at 80% power was computed for each cluster count. The analysis indicates that 56 clusters are required to detect a 5-percentage-point effect at 80% power under this variance structure, a requirement that exceeds the 32-cluster maximum in the design grid. The contribution is a design diagnostic: it specifies the cluster counts, effect sizes and variance conditions under which a threshold-based evaluation of warehouse receipt collateral can credibly identify storage-duration effects, and it identifies the conditions under which such an evaluation would be underpowered.