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能夠看到多遠(yuǎn)的范圍，無(wú)論是cost，time，scope還是其他的uncertainty
(2)Project duration
(3)People，通常關(guān)系到people的experiences和隨之而來(lái)的bias
(4)Project structure and organisation
(5)Padding estimate，通常為了穩(wěn)妥起見(jiàn)，都會(huì)選擇在預(yù)估的結(jié)果上增加一些padding來(lái)對(duì)沖風(fēng)險(xiǎn)，畢竟leader愿意聽(tīng)的是提前完成項(xiàng)目而不是預(yù)計(jì)要延期
(6)Organisation culture，團(tuán)隊(duì)之間處于什么樣的階段也很重要，是互相爭(zhēng)執(zhí)不理解，還是把目光都放在如何更好的完成項(xiàng)目上，對(duì)預(yù)估結(jié)果影響很大
(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的方法更加宏觀，適合不確定性強(qiáng)的項(xiàng)目。
2.1.1 Consensus methods
最常見(jiàn)的方法是讓很多專家共同開(kāi)會(huì)討論，最終達(dá)到共識(shí)；為了避免bias，很多時(shí)候也使用匿名打分解釋原因的“Delphi Method”方法
2.1.2 Ratio methods
根據(jù)過(guò)往經(jīng)驗(yàn)推測(cè)預(yù)估，根據(jù)部分預(yù)估整體
2.1.3 Appotionment methods（分配方法）
是ratio方法的一種擴(kuò)展，適合比較規(guī)范的項(xiàng)目，根據(jù)過(guò)往項(xiàng)目經(jīng)驗(yàn)分配預(yù)估
2.1.4 Function points methods
通常在IT行業(yè)常見(jiàn)，根據(jù)功能評(píng)分來(lái)預(yù)估
2.1.5 Learning curves
適合多次重復(fù)的任務(wù)，適合人力繁重的項(xiàng)目，隨著次數(shù)的增多，處理的時(shí)間會(huì)越來(lái)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的方法更加微觀，適合必須滿足某些條件的項(xiàng)目。
2.2.1 Template methods
以過(guò)往項(xiàng)目為基礎(chǔ)，不同的地方進(jìn)行調(diào)整，與前面ratio等方法類似，不同的是這種方法是從底層向上推演，根據(jù)比較確定的內(nèi)容預(yù)估
2.2.2 Parametric procedures applied to specific tasks
和ratio方法更加相似，也是從部分推整體。
2.2.3 Range estimating
從單一打分轉(zhuǎn)變?yōu)閞ange打分，根據(jù)range大小能判別風(fēng)險(xiǎn)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
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.
