什么是项目管理三角？

铁三角项目模型的某些约束被认为是“钢铁般的”，因为你无法在不影响其他约束的情况下改变一个约束最初的铁三角项目管理是马丁·巴恩斯博士在1969年提出的，遵循瀑布式的产品开发方法：范围是固定的，而资源和时间是可变的。对于软件团队来说，这意味着团队通过定义产品要求来确定项目的范围（工作项列表）来启动项目。资源和时间表是可变的，根据固定范围进行估计。

铁三角项目管理的目的是为产品团队提供必要的信息，以便做出有助于业务的权衡。例如，如果团队面对的是一个固定的范围，他们可能会在项目进行到一半时意识到无法按期发布。他们只能控制下面的几个变量：1) 时间 - 他们可以接受发布日期推迟或 2) 资源 - 他们可以为项目添加更多人员，但这将增加成本。随着 21 世纪软件开发的发展，需要更好的协作和快速响应客户反馈的能力变得至关重要，因此，敏捷方法应运而生。

If your team practices waterfall project management or are new to agile development, the important thing to remember is the difference between what is fixed and what is estimated. Unlike waterfall development, agile projects have a fixed schedule and resources while the scope varies. While the scope of a project might change in agile development, teams commit to fixed iterations of work: sprints if you're using a scrum framework and WIP limits if you're using a kanban framework. It's also a best practice to keep teams fixed throughout the development process. By keeping teams consistent on a product or project, they become more efficient through developed trust and continuity.

The idea of scope is the same in agile development: what software to build and deliver. However, agile focuses on high-level requirements rather than trying to come with deep and detailed requirements upfront. The scope of a project gets regularly managed and groomed (prioritized) by the product manager in a tool like Jira. The product manager decides which work should be accomplished in the next sprint based on agile qualitative and quantitative feedback from various channels (market conditions, customer feedback, competitions, etc..). And because resources and time are fixed, it's easier for development teams to react to market changes and to deliver value to customers faster. This transparency of constraints keeps teams honest about a consistent and fast release cadence, which is a key tenant of agile development; and by looking at projects through the lens of the project management triangle teams are able to adapt without abandoning a plan.

> > Set up your first agile project with this interactive tutorial

As projects become bigger, more teams are needed and the time box gets longer. Thus, the notion of fixing resources and time, while scope varies, is not a valid approach for all agile projects. Long-term agile planning requires a more flexible project management triangle that allows teams to plan ahead and ensures that they're meeting the business objectives. Think for instance about the lean startup movement, and the notion of a minimum viable product (MVP). An MVP by definition is a small set of features (scope) that delivers customer value. To get to that MVP, teams might need to stick to a fixed scope – the number of features – with time being their only variable (e.g. you can't release without certain features, so the release date gets pushed). Only after launching the MVP, teams switch to a variable scope.

Regardless of the differences between waterfall and agile development, when using the project triangle, there’s no right or wrong way. It's there to help you make the best decisions and trade-offs to reach your business goals. A tool like Timelines visualizes the building blocks of a plan – scope, people, and time – to help teams plan in real-time. You can easily play with scope, teams, and time to plan your next product release, using the team's existing data in Jira.