Estimating project times and costs

Estimating: The process of forecasting or approximating the time and cost of completing project deliverables; The task of balancing expectations of stakeholders and need for control while the project is implemented.

1. Factors influencing the quality of estimates

(1)Planning horizon, PM能够看到多远的范围，无论是cost，time，scope还是其他的uncertainty
(2)Project duration
(3)People，通常关系到people的experiences和随之而来的bias
(4)Project structure and organisation
(5)Padding estimate，通常为了稳妥起见，都会选择在预估的结果上增加一些padding来对冲风险，毕竟leader愿意听的是提前完成项目而不是预计要延期
(6)Organisation culture，团队之间处于什么样的阶段也很重要，是互相争执不理解，还是把目光都放在如何更好的完成项目上，对预估结果影响很大
(7)Other (non-project) factor

2. Types of estimates

2.1 Top down (macro)

are usually derived from someone who uses experience and/or information to determine the project duration and total cost；are made by top managers who have little knowledge of the processes used to complete the project
Top down的方法更加宏观，适合不确定性强的项目。

2.1.1 Consensus methods

最常见的方法是让很多专家共同开会讨论，最终达到共识；为了避免bias，很多时候也使用匿名打分解释原因的“Delphi Method”方法

2.1.2 Ratio methods

根据过往经验推测预估，根据部分预估整体

2.1.3 Appotionment methods（分配方法）

是ratio方法的一种扩展，适合比较规范的项目，根据过往项目经验分配预估

2.1.4 Function points methods

通常在IT行业常见，根据功能评分来预估

2.1.5 Learning curves

适合多次重复的任务，适合人力繁重的项目，随着次数的增多，处理的时间会越来越短

2.2 Bottom up (micro)

can serve as a check on cost elements in the WBS by rolling up the work packages and associated cost accounts to major deliverables at the work package level
Bottom up的方法更加微观，适合必须满足某些条件的项目。

2.2.1 Template methods

以过往项目为基础，不同的地方进行调整，与前面ratio等方法类似，不同的是这种方法是从底层向上推演，根据比较确定的内容预估

2.2.2 Parametric procedures applied to specific tasks

和ratio方法更加相似，也是从部分推整体。

2.2.3 Range estimating

从单一打分转变为range打分，根据range大小能判别风险大小

2.3 Hybird: phase estimating

Uses a two-estimate system over the life of the project；
A detailed (micro) estimate is developed for the immediate phase；
A macro estimate is made for the remaining phases of the project

Topdown and bottomup comparison

3. Estimating projects: preferred approach

(1)Make rough top-down estimates.
(2)Develop the WBS/OBS.
(3)Make bottom-up estimates.
(4)Develop schedules and budgets.
(5)Reconcile differences between top-down and bottom-up estimates.