The best way I’ve found for agile teams to estimate is by playing Planning Poker® (Grenning 2002). Planning Poker® estimating combines expert opinion, analogy, and disaggregation into an enjoyable approach to estimating that results in quick but reliable estimates.
Participants in Planning Poker® include all of the developers on the team. Remember that developers refers to all programmers, testers, database engineers, analysts, user interaction designers, and so on. On an agile project, this will typically not exceed ten people. If it does, it is usually best to split into two teams. Each team can then estimate independently, which will keep the size down. The product owner participates in Planning Poker® estimating but does not estimate.
At the start of Planning Poker®, each estimator is given a deck of cards. Each card has written on it one of the valid estimates. Each estimator may, for example, be given a deck of cards that reads 0, 1, 2, 3, 5, 8, 13, 20, 40, and 100. The cards should be prepared prior to the Planning Poker® meeting, and the numbers should be large enough to see across a table. Cards can be saved and used for the next planning poker session.
For each user story or theme to be estimated, a moderator reads the description. The moderator is usually the product owner or an analyst. However, the moderator can be anyone, as there is no special privilege associated with the role. The product owner answers any questions that the estimators have. However, everyone should remember that at some point additional discussion does not lead to improved accuracy. The goal in Planning Poker® estimating is not to derive an estimate that will withstand all future scrutiny. Rather, the goal is to be somewhere well on the left of the effort line, where a valuable estimate can be arrived at cheaply.
After all questions are answered, each estimator privately selects a card representing his or her estimate. Cards are not shown until each estimator has made a selection. At that time, all cards are simultaneously turned over and shown so that all participants can see each estimate.
It is very likely at this point that the estimates will differ significantly. This is actually good news. If estimates differ, the high and low estimators explain their estimates. It’s important that this does not come across as attacking those estimators. Instead, you want to learn what they were thinking about.
As an example, the high estimator may say, “Well, to test this story, we need to create a mock database object. That might take us a day. Also, I’m not sure if our standard compression algorithm will work, and we may need to write one that is more memory efficient.” The low estimator may respond, “I was thinking we’d store that information in an XML file—that would be easier than a database for us. Also, I didn’t think about having more data—maybe that will be a problem.
The group can discuss the story and their estimates for a few more minutes. The moderator can take any notes she thinks will be helpful when this story is being programmed and tested. After the discussion, each estimator re-estimates by selecting a card. Cards are once again kept private until everyone has estimated, at which point they are turned over at the same time.
In many cases, the estimates will already converge by the second round. But if they have not, continue to repeat the process. The goal is for the estimators to converge on a single estimate that can be used for the story. It rarely takes more than three rounds, but continue the process as long as estimates are moving closer together. It isn’t necessary that everyone in the room turns over a card with exactly the same estimate written down. If I’m moderating an estimation meeting, and on the second round four estimators tell me 5, 5, 5, and 3, I will ask the low estimator if she is OK with an estimate of 5. Again, the point is not absolute precision but reasonableness.
The Right Amount of Discussion
Some amount of preliminary design discussion is necessary and appropriate when estimating. However, spending too much time on design discussions is often wasted effort. Here’s an effective way to encourage some amount of discussion but make sure that it doesn’t go on too long.
Buy a two-minute sand timer, and place it in the middle of the table where planning poker is being played. Anyone in the meeting can turn the timer over at any time. When the sand runs out (in two minutes), the next round of cards is played. If agreement isn’t reached, the discussion can continue. But someone can immediately turn the timer over, again limiting the discussion to two minutes. The timer rarely needs to be turned over more than twice. Over time this helps teams learn to estimate more rapidly.
It is possible to play Planning Poker® with a subset of the team, rather than involving everyone. This isn’t ideal but may be a reasonable option, especially if there are many, many items to be estimated, as can happen at the start of a new project.
The best way to do this is to split the larger team into two or three smaller teams, each of which must have at least three estimators. It is important that each of the teams estimates consistently. What your team calls three story points or ideal days had better be consistent with what my team calls the same. To achieve this, start all teams together in a joint Planning Poker® session for an hour or so. Have them estimate ten to twenty stories. Then make sure each team has a copy of these stories and their estimates and that they use them as baselines for estimating the stories they are given to estimate.
When to Play Planning Poker® Estimating
Teams will need to play Planning Poker® at two different times. First, there will usually be an effort to estimate a large number of items before the project officially begins or during its first iterations. Estimating an initial set of user stories may take a team two or three meetings of from one to three hours each. Naturally, this will depend on how many items there are to estimate, the size of the team, and the product owner’s ability to clarify the requirements succinctly.
Second, teams will need to put forth some ongoing effort to estimate any new stories that are identified during an iteration. One way to do this is to plan to hold a very short estimation meeting near the end of each iteration. Normally, this is quite sufficient for estimating any work that came in during the iteration, and it allows new work to be considered in the prioritization of the coming iteration.
Alternatively, Kent Beck suggests hanging an envelope on the wall with all new stories placed in the envelope. As individuals have a few spare minutes, they will grab a story or two from the envelope and estimate them. Teams will establish a rule for themselves, typically that all stories must be estimated by the end of the day or by the end of the iteration. I like the idea of hanging an envelope on the wall to contain unestimated stories. However, I’d prefer that when someone has a few spare minutes to devote to estimating, he find at least one other person and that they estimate jointly.
Why Planning Poker® Works
Now that I’ve described Planning Poker®, it’s worth spending a moment on some of the reasons why it works so well.
First, Planning Poker® brings together multiple expert opinions to do the estimating. Because these experts form a cross-functional team from all disciplines on a software project, they are better suited to the estimation task than anyone else. After completing a thorough review of the literature on software estimation, Jørgensen (2004) concluded that “the people most competent in solving the task should estimate it.
Second, a lively dialogue ensues during planning poker, and estimators are called upon by their peers to justify their estimates. This has been found to improve the accuracy of the estimate, especially on items with large amounts of uncertainty (Hagafors and Brehmer 1983). Being asked to justify estimates has also been shown to result in estimates that better compensate for missing information (Brenner et al. 1996). This is important on an agile project because the user stories being estimated are often intentionally vague.
Third, studies have shown that averaging individual estimates leads to better results (Hoest and Wohlin 1998) as do group discussions of estimates (Jørgensen and Moløkken 2002). Group discussion is the basis of Planning Poker ®, and those discussions lead to an averaging of sorts of the individual estimates.
Finally, Planning Poker® works because it’s fun.
—From Agile Estimating and Planning by Mike Cohn
Brenner, Lyle A., Derek J. Koehler, and Amos Tversky. 1996. “On the Evaluation of One-sided Evidence.” Journal of Behavioral Decision Making 9:59–70.
Grenning, James. 2002. Planning Poker, http://renaissancesoftware.net/papers/44-planing-poker.html
Hagafors, R., and B. Brehmer. 1983. “Does Having to Justify One’s Decisions Change the Nature of the Decision Process?” Organizational Behavior and Human Performance 31:223–232.
Hoest, Martin, and Claes Wohlin. 1998. “An Experimental Study of Individual Subjective Effort Estimations and Combinations of the Estimates.” Proceedings of the 20th International Conference on Software Engineering, pp. 332–339.
Jørgensen, Magne. 2004. “A Review of Studies on Expert Estimation of Software Development Effort.” Journal of Systems and Software 70(1–2):37–60.
Jørgensen, Magne, and Kjetil Moløkken. 2002. “Combination of Software Development Effort Prediction Intervals: Why, When and How?” Fourteenth IEEE Conference on Software Engineering and Knowledge Engineering.