Abstract
Robustness of binary incomplete block designs against giving rise to a disconnected design in the event of observation loss is investigated. A link is established between the E-value of a planned design and the extent of observation loss that can be experienced whilst still guaranteeing an eventual design from which all treatment contrasts can be estimated. Patterns of missing observations covered include loss of entire blocks and loss of individual observations. Simple bounds are provided enabling practitioners to easily assess the robustness of a planned design.