How to do Critical Path analysis? - YouTube

Greetings! In this lesson I'm going to demonstrate how to do the forward and

backward path to determine the estimated duration of the project. So here we have

exercise 9 the air control company project and the first thing you should

do when you look at a diagram like this is review what's on the legend and the

legend contains early start, early finish, late start, late finish, duration, slack

and the ID right now all we have is the ID and the duration will be doing the

forward and backward pass to get the early start and the late finish

information so to begin this exercise we start with an early start of 0 now I

don't want to argue about this some people want to get into existential

questions about how can it really start be a zero but just to do the methodology

the uses technique we always start with zero just accept that and make life a

lot easier so now we simply add and move do what all the text calls the forward

pass and so the earliest that activity a if it takes two days it will be done on

the second day that's your earliest it can be done then B can start once a is

done so can see so c, so can D and so can E. the earliest start is the early finish

for the preceding activity now we simply add Early start to the duration, two

and ten is 12 - for the preceding activity. 2 and 13 15 2 and 18 is 20 ok now we move to the next activity that

we can look at and that's F and F is a merge activity which means there's more

than one activity that has to be completed before F can start and this is

where you need to keep in mind choosing the larger number so the we

have two early finishes 12 and 15 that didn't come out too well so I'll

correct that and we choose the larger number which is 15 because F can't start

until both C and D are completed C's done in the 12th but DS not to the 15th

so f has to wait until DS done the earliest it can start then is 15 15 and

15 is 30 and now we have another merge G can't start until both B and F er done

early as B is done as the 17 the earliest F is done is 30 again we choose

the larger number 30 30 and 10 is 40 and then finally we have the last activity

is also a merge activity H can't start until both G and E are done the earliest

G is done is 40 the earliest G is done is 20 we choose the larger number 40 40

+ 5 is 45 and we would now say that the earliest time this project could be

completed is in 45 days assuming days is the time unit that we're using now the

late finish we go backwards and this time we subtract and instead of choosing

the larger number we choose the smallest number when we're forced to make a

choice so the latest H can finish without

delaying the project obviously is 45:46 would mean it's a day late so the late

finish and the early finish for the last activity are always the same we subtract

5 from 45 and the late start for H is 40 again the late start and the late finish

is the latest this activity can start without delaying the project the latest

this activity can finish without delaying the project so now we move back

we simply transfer the late start to the late finish of the preceding activity 40

10 minus 40 is 30 30 into here is 15 15 minus 30 is 15 we go down here

we can take care of e 40 18 minus 40 is remember about your math 22 okay now we

can begin to see there's a difference here this is a non critical activity we

look up at B latest finish for B without delaying G is 30 15 minus 30 is 15 and

these two activities off the merge activity it's both 15 and 15 you've got

to follow the arrows backwards 13 minus 15 is 2 10 minus 15 is 5 and then we've

got a bunch of arrows out of activity a we got a late start of 20 to a late

start of two late start of 5 and the late start at 15 we choose the smaller

number to 2 minus 2 is obviously 0 and when you have that you know you're in

good shape and now we subtract to determine the slack how much an activity

can be delayed without delaying the project we subtract the early start from

the late start and the early finish from the late finish on these problems

they'll be identical because there's no lags we'll talk about legs and I think

the third problem but for now we do both and they should be identical so we go up

here subtract 2 from 22 it's 20 so what this means is that activity e

can be delayed the start of activity can be delayed up to 20 days without

delaying the project and that makes sense if you look that activity II has

an early start of 2 and a late start of 22 the difference between that is 20 so

it has quite a bit of slack and lo and behold when we strike subtract the two

early finish and late finish we get the event achill 20

here we have zeros which means delay and activity D will delay the project this

is an indication that activity D is on the critical path and we have three and

two from 15 is 13 and then we go here subtract those two

we got zeros zeros up here and then the last activity actually has euros and

then we trace the critical path follow the zeros in the slack in this case we

have one critical path I like to highlight it make it stick out and so ad

F G H is the critical path a delay in the duration the estimated duration of

any one of these five activities will delay the project if activity D takes 15

days instead of 13 days it will add two days to the duration of the project

if however activity B instead of taking 15 days end up taking 20 days it would

not delay the duration of the project or the completion of the project I should

say because it has a slack of 13 now the one other thing that we talked about is

assessing the sensitivity of the network and that's the likelihood that the

original critical path will change once you start the project and that's a

function of two things one the number of critical paths and 2 how much slack the

non-critical activities have here we only have one critical path which

suggests it's not very sensitive but we do have one activity C that only has

three days of slack the other activities have quite a bit of slack B and E so you

would say that this is moderately sensitive and our only concern is that

if C is delayed it could become critical so remember add and choose the larger

number going forward when you go back subtract and choose a smaller number