This month’s IIBAâ BA Connections newsletter has a great article by Elizabeth Larson, titled "Partnering for Success: An IIBA/PMI Joint Collaboration", on the relationship and responsibilities between project managers (PM) and business analysts (BA).  As Ms. Larson correctly portrays, often this relationship can be viewed as adversarial and competitive.  The truth is that it can be a very successful partnership when managed correctly.
One particular point that rings true with me is her comment on the trust relationship between the BA and the business.  Trust is essential to the goal of requirements elicitation, which is "to ensure that a stakeholder’s actual underlying needs are understood"1.  In order to build and maintain that trust, an analyst needs to provide an open dialogue, with limited censorship, for the business to communicate their needs.  This allows them to accurately capture the breadth and depth of the requirements before subjecting them to the scrutiny of budget and schedule, among other considerations. 
Ultimately, it is on both the PM and the BA to ensure that a project succeeds.  By incorporating some of the boundaries established in this article, I have been able to jointly partner with my PM on scope management.  This in turn has allowed me to ensure that my focus remains on customer advocacy and product quality, while letting my PM peer maintain ownership for budget and schedule.  Call it divide-and-conquer!  After all, even an under-budget, on-time delivery won’t be accepted if the core business requirements aren’t met or vice versa.
A Guide to the Business Analysis Body of Knowledgeâ (BABOKâ Guide), 2.0 ed., International Institute of Business Analysis, Toronto, Ontario, Canada, 2009, pg. 7