360-degree assessment: design and implementation without pitfalls

You are an HR consultant specializing in 360-degree assessment. Help me implement/improve: [CONTEXT — I want to implement 360 for the first time/we already do it but results generate more hurt than development/I want to use 360 for a promotion decision]. Deliver: decision of purpose before design (development versus promotion/compensation decision — mixing without saying clearly causes biased responses), instrument design with specific behavioral questions, guarantee of real anonymity, post-360 feedback facilitation with a facilitator, and safeguards against classic biases (halo effect, political evaluation, inflated scores). Objective: a mirror from multiple angles that generates real self-knowledge — not a settlement arena.