比幅圖你,
點樣搵番晒
ES, EF, LS & LF 呢?
1) Define Critical
path: longest path, shortest time, zero float
1.Start -> A -> B -> C-> End,
duration: 31 days. ç Critical Path because the longest period
2.Start ->D -> E ->F -> End,
duration: 18 days.
3.Start -> D -> B -> C -> End,
duration: 26 days.
4.Start -> G ->H ->I -> End,
duration: 13 days.
5.Start -> G -> E ->F -> End,
duration: 16 days.
2) Calculate
Early Start (ES) and Early Finish (EF)
Using Forward Pass, i.e. start from the
beginning and proceed to the end
Formula [for example starts with 1]:
·
Early
Start of the activity = Early Finish of predecessor activity + 1
·
Early
Finish of activity = Early Start of activity + Activity duration – 1
Calculation of A -> B -> C
On a critical path, Early Start, and Early
Finish dates will be the same as Late Start and Late Finish dates.
ES (A) = 1 (Since this is the first
activity of the path)
EF (A) = ES (A) + activity duration – 1 = 1
+ 10 – 1 = 10
ES (B) = EF of predecessor activity + 1 =
10 + 1 = 11
EF (B) = ES (B) + activity duration – 1 =
11 + 12 – 1 = 22
ES (C) = EF of predecessor activity + 1 =
22 +1 = 23
EF (C) = ES (C) + activity duration – 1 =
23 + 9 – 1 = 31
Calculation of D -> E -> F
ES (D) = 1 (Since this is the first
activity of the path)
EF (D) = 1 + 5 – 1 = 5
@ Trick: Activity E has two predecessor,
which one should select?
Select the activity with the greater
Early Finish date. EF (D) is 5 while EF(G) is 3, so use D
ES (E) = EF of predecessor activity + 1 = 5
+ 1 = 6
EF (E) = 6 + 7 – 1 = 12
ES (F) = 12 + 1 = 13
EF (F) = 13 + 6 -1 = 18
Calculation of G -> H -> I
ES (G) = 1 (Since this is the first
activity of the path)
EF (G) = 1 + 3 – 1 = 3
ES (H) = 3 + 1 = 4
EF (H) = 4 + 4 – 1 = 7
ES (I) = 7 +1 = 8
EF (I) = 8 + 6 – 1 = 13
3) Calculating Late Start (LS) and Late Finish
(LF)
Using Backward Pass, i.e. start from the
end back to the beginning
Formula [for example starts with 1]:
·
Late
Start of Activity = Late Finish of activity – activity duration + 1
·
Late
Finish of Activity = Late Start of successor activity – 1
Calculation of A -> B -> C
@ On a critical path, ES and EF dates will
be the same as LS and LF dates.
所以直接將ES è LS,
EF è LF copy落去就得
Calculation of D -> E -> F
LF (F) = 31 (because you cannot allow any
activity to cross the project completion date)
LS (F) = 31 – 6 +1 = 26
LF (E) = 26 – 1 = 25
LS (E) = 25 – 7 + 1 = 19
@ Trick: Activity D has two successor
activities, which one should use?
Select the lower Late Start date. LS (B) =
11, LS (E) = 19, so choose B
LF (D) = 11 – 1 = 10
LS (D) = 10 – 5 + 1 = 6
Calculation of G -> H -> I
LF (I) = 31 (because you cannot allow any
activity to cross the project completion date)
LS (I) = 31 – 6 + 1 = 26
LF (H) = 26 – 1 = 25
LS (H) = 25 – 4 + 1 = 22
LF (G) = 19 – 1 = 18 (choose E not H
because LS (E) has lower Late Start date)
LS (G) = 18 – 3 + 1 = 16
4)
Answer
Question
1)
What is the waiting period of
activity D-E-F?
i.e. 即係要計float (stack). In
examination, float always means total float.
Answer:
Critical path =
A-B-C = 31 days
D-E-F = 18 days
Total float of
D-E-F
= Critical Path – active activity
= 31 – 18 = 13
2)
What is the total float (slack)
of Activity (E) & (D)?
Answer:
Method
1:
Total float of (E)
= LS - ES
=
19 – 6 = 13
Method 2:
Total float of (E)
= LF – EF
= 25 – 12 = 13
Method
1:
Total float of
(D) = LS - ES
=
6 – 1 = 5
Method 2:
Total float of (D)
= LF – EF
= 10 -5 = 5
3)
What is the free float of
Activity (D) & (G)?
@ Both activity D & G can have the free float because Activity D
and G are converging on one common activity.
@ Activity A 都有converging, 點解唔計佢? 因為佢係Critical
Path activity
Answer:
Free
float of (D) = ES of successor – EF of current activity – 1
=
11 – 5 – 1 = 5
Free
float of (G) = ES of successor – EF of current activity – 1
=
6 – 3 – 1 = 2
4)
Which start date should be
chosen? “1” or “0”?
the first day of
the project can be “1” or “0”, both conventions are correct
reason to choose
project start from “1”:
l This convention is followed the PMBOK Guide.
l It seems more logical to me to say, “Hey, today is my first day of
the project!” instead of saying, “Hey, today is my zero day of the project.”
5)
會唔會有多過1條 Critical Path?
Yes. 因為大家做嘅野雖然唔同, 但時間可以一齊完成.
6)
Critical Path 會唔會變?
Yes. 比如有change request 要change scope / time / cost, 導致某啲activity 啲時間唔同左, 或者某啲 activity delay 左, 令到成條activity chain 長左, 都會變左條Critical Path.
7)
會唔會有負數total float
(slack)?
Planning 嘅時候唔會有. 但行到中間, 有啲job delay 左, Early Finish 因而加大左, Late Finish 不變, 所以 LF – EF 就會出要負數
8)
如果Activity (D) 用左slack, 後面E & F 仲有無slack?
無. 因為slack 係成條activity
path share 的. 前面activity 用左slack 而late start/finish 左, 日期上巳迫近左後面activity 嘅late start/finish, 唔會再有slack 走棧.
9)
剩係睇 Critical Path 得唔得?
唔得. 因為cricital path 會變, non-critical path 因為scope change / delay
都有機會變成critical path. 所以both critical-path 和 non critical-path 都要睇.
Reference:
PMBOK 5th Edition
[this example starts with 1, formula needs
+/- 1]
