## 2 Background
### Concept of Estimation Life Cycle  
A common mistake is to consider estimation as a one-time activity that takes place before the project start. As a rule, such a mistake leads to a low-quality estimate that, in turn, causes the unpredictability of project resources and deadlines. The key to avoiding this issue is to organize the estimation as a multistep process taking place at each SDLC stage. Of course, depending on the project life cycle stage, the estimation targets different goals, requires various methods, and satisfies certain accuracies. In the current paper, the estimate accuracy means closeness of the estimated values to the corresponding actual value requirements; obviously, the accuracy defined in this way can be measured only after receiving the actual values, i.e., during the project implementation or even after its completion. In Figure 1, the concept of the estimation life cycle (ELC) proposed by the authors is visualized. Being based on the cone-of-uncertainty idea [1], the concept represents estimation as an integral part of SDLC. The estimation starts from the so-called ideation phase (which is a very early stage of a project), when the level of uncertainty in the understanding of different aspects of the project is the highest. As a rule, the estimates at this stage are represented as wide ranges that rather express a rough order of the required resources and time. In the project ideation stage, introductory and preliminary estimates are undertaken. The main difference between these successive steps is that introductory estimates are mostly based on similarity with past projects and do not need a project scope analysis. In its turn, the preliminary estimation requires the project scope analysis. Definitely, such estimate types are rather informative and not recommended for commitments (e.g., when signing a contract). The estimation goal at the project ideation stage is to roughly understand the project scope and resources required for the project's implementation while minimizing the efforts to prepare the estimate itself. As the level of uncertainty decreases, it becomes possible to provide more accurate estimates using the outcomes from the previous stages as inputs. Higher accuracy of intermediate estimates is mostly achieved through the elaboration of the project scope as well as distinguishing the project implementation phases. In order to reduce the efforts spent on the estimation itself, the corresponding methods do not consider dependencies among the project tasks. Although their definitiveness is higher, intermediate estimates are still not reliable enough to be used for commitments. Further information about intermediate estimation can be found in [9; 10]. Figure 1: Concept of estimation life cycle. After a certain amount of analysis and design, providing a precise estimate and a project release plan becomes possible. Unlike in the previous stages, the precise estimate's accuracy is acceptable to use to make commitments. In this regard, precise estimation is similar to what is called the definitive estimate [13]. Since the efforts required to prepare such an estimate are greater in comparison to the previous stages, it is worth undertaking it in the case of a high probability of the project implementation's start. In the project implementation phase, it is important to constantly monitor progress, comparing the actual efforts with the estimate and reacting proactively to undesirable deviations from the release plan. To avoid missing deadlines, regular re-estimation of the remaining project tasks is necessary. As this part of the project life cycle is likely to follow an agile methodology, the agile team is the main source of feedback on the estimate, and the team is responsible for adjusting the estimate according to the actual project state. After project completion, it is quite useful to finalize the feedback on the estimates comparing them with the actual working time as well as analyzing lessons learned. The collected feedback is valuable for future project estimation and planning.