As the project manager, you aren't responsible for completing every task. What you are responsible for is identifying and helping assign those tasks, and then estimating how long they'll take to complete. These estimates come together to determine the overall project schedule. So, how do you estimate the amount of time a given task will take? You do this with the help of your team. Time estimation is a prediction of the total amount of time required to complete a task. Effort estimation is a prediction of the amount and difficulty of active work required to complete a task. Effort estimation differs from time estimation in that effort quantifies the amount of time it will take a person to complete work on a task. On the flip side, time refers to the overall duration of the task from start to finish. That includes inactive time. Here's an example. The effort estimation for painting a wall might be 30 minutes, but time estimation might be 24 hours. That's because in addition to the 30 minutes of active painting time, there are also 23 and a half hours of inactive drying time. It's important to understand the difference between time estimation and effort estimation, because it can help you be more efficient with your available resources. If there's idle time baked into a given task, your teammate is effectively free to do other things. A painter can do other tasks while the wall is drying, like painting the mailbox or the window trim. An unrealistic effort estimate can negatively impact a project schedule. Generally, this happens when you underestimate the amount of time it will take to complete a task. Often, the culprit for under estimating effort is optimism. And listen, optimism is a wonderful trait for a project manager to possess. But too much optimism can lead you to overlook potential risks that could set your plans behind schedule. Though it might be tempting to make the optimistic assumption that tasks will be executed exactly according to plan, there is always a possibility that there will be setbacks. So, how do you try to avoid making unrealistic effort estimates? You can do this by communicating with teammates assigned to each task. Your teammates will have the most realistic understanding of the amount of work required to complete a task and should be able to provide you with the best estimate. Let's imagine this scenario in the context of our Plant Pals project at Office Green. As a reminder, you're launching a new service to provide top office green customers with small, low-maintenance plants that they can place on their desks. You might assume that creating a contact list of top customers is relatively straightforward and can be completed in a single day. But it's important to really consider certain sub-tasks required to complete work in your planning. Sub-tasks refer to smaller tasks that are required to complete a larger task. For example, this might include meeting with the global sales team to identify clients gathering contact information, determining client language preferences, and building a spreadsheet to house this information. Asking the teammate assigned to the task for their estimate is likely to yield a more accurate estimation since they'll have a deeper understanding of the work and the nuances of what's required to complete the task. You might learn that creating the contact list may take two days to complete, which could be double the time you originally expected. Of course, you can usually ask follow-up questions, or even gently push back on their estimate, as needed. Later on, we'll discuss more of the techniques you can use to get more accurate estimates from your teammates. Now, even though task owners tend to have the strongest sense of how much time they'll need to complete a task, the fact is that effort estimates are just that, estimates, meaning that sometimes those estimates won't be accurate. For example, in our Plant Pals scenario, your teammate estimates that it will take two days to create a contact list of top customers. But, let's say that the Sales team is out of the office for a team-building exercise and unable to meet about the client list until after the weekend. This will create a task delay, and as a result, the original estimate is no longer accurate. Luckily, there's a helpful tool called a buffer that you can use during the planning phase to protect against inaccurate effort estimates. A buffer is extra time added to the end of a task or a project to account for unexpected slowdowns or delays in work progress. Buffers are important because they can provide some leeway, just in case your time and effort estimates turn out to fall a bit short. With a buffer, you can add extra time into your schedule, and your project shouldn't fall off track when task delays inevitably arise. There are two types of buffers you can use when planning your schedule: task buffers and project buffers. First, we have task buffers, which refer to extra time tacked on to a specific task. Task buffers should be used primarily for tasks that are out of the project team's control. For example, you might ask a potential plant vendor to provide you with a cost estimate by Monday. You might assign them this deadline, knowing that you won't actually need the estimate until Thursday. The time between Monday and Thursday is your buffer, and it provides your team with extra time just in case the vendor sends their estimates to you a day or two late. Task buffers should be used more sparingly for tasks within the project team's control. For example, you might choose to add buffers only to tasks that are difficult to complete or that have an element of unpredictability, like the length of time it will take plants to grow. Adding a buffer to every task could lengthen your project schedule unnecessarily, leaving you, your team, and your stakeholders with an unrealistic timeline. This is where project buffers come in handy. Project buffers differ from task buffers in that they provide extra time to the overall project schedule. Rather than adding a buffer to every task, you can add extra time as a buffer towards the end of your project schedule. Then you can use that extra time, two to three days, for example, as needed throughout the project. For instance, if a teammate misses a deadline here and there, the project buffer gives you space in the overall schedule to make up for lost time. I use buffers often in my day-to-day role at Google. For example, on a recent project at Google, I was working with a new hire who was great at coding but kept missing deadlines. I realized they weren't giving themselves enough buffer time to do testing. I started to ask questions about their current workload and the complexity of their tasks, and based on their answers to those questions, I was able to gather insights about their work and determine where I needed to add buffer to their tasks. Ultimately, my goal is to ensure that I'm setting a realistic timeline for the project. After all, if you hit your project goal two months later than expected, your organization may not consider the project a success. Time estimation, effort estimation, and buffers can help you build realistic plans for reaching the project goal.