Abstract: The traditional assemble-to-order system has been widely used in the manufacturing industry, and any multi-item (component) inventory system in which customer orders may consist of several items (components) in different amounts can be viewed as generalized ATO system. A generalized assemble-to-order supply chain, which is composed of multi-suppliers, single manufacturer and retailer, is considered. In the context of uncertain components (or items) supplying in the upstream and uncertain demand in the downstream, present three group buying models:base model, standard model and coordination model. Due to the uncertainty of the supply chain and complexity of group buying models, the methodology of mathematic deduction and Excel VBA programming are used to simulate data example analysis. These three group buying models' impact on manufacturer, retailer and global supply chain's performance is compared. In the group buying base model, the retailer itself offers price discount to customer, which can increase the order quantity to manufacturer and is always beneficial to manufacturer, but is not to retailer. In the group buying standard model, the manufacturer need offer wholesale price quantity discount contract to retailer before group buying beginning, which can incentive retailer to increase the order quantity by group buying and alleviate the uncertainty of group buying demand. And the standard model which is dominated by manufacturer is better than base model under some certain conditions, but is probably to make retailer even manufacturer itself suffering the loss of profit. In these two models, the manufacturer and the retailer make decisions and initialize group buying dispersedly, and they face the risks of uncertainty of components' supply, order from retailer, manufacturer's yield and customer's demand. Finally group buying coordination model, in which manufacturer and the retailer make centralized decisions by sharing demand information from retailer and yield information from manufacturer, is presented. The study result shows:group buying coordination model is optimal among these three models, and can increase supply chain's global profit, the Pareto's improvement is achieved; but the coordination's performance is bounded by customers' structure of group buying.

Key words: generalized assemble-to-order, group buying, supply chain coordination