I have recently been approached to help with an answer to the following PMP or PMI-SP type of question:
Activity A has a duration of 5, Activity B has a duration of 10, they are connected with a SF link from A to B (no lag). What is the total float for Activity A?
The basic construct of any link is that lags (or leads) create overlap. If the link was a FS link from B to A, B would finish at the end of time period ‘10’ A would start at the beginning of ‘11’. Therefore changing to a SF link, the start of A on ‘11’ is allowed because B finishes at the end of ‘10’. This makes sense, if permanent power is connected ready for use on Thursday morning; you can finish with the temporary generators on Wednesday night (unless you choose to specify an overlap, ie impose a lag).
However, the function of a link is to control the performance of the successor (not the predecessor). This raises a number of logical possibilities.
Option 1: Activity A has no predecessors, therefore it should start at ‘1’ and no successors therefore it should finish at ‘5’ and this applies to both the forward pass and the back pass meaning the Total Float is 0.
Option 2: Activity A has no predecessors, therefore it should start at ‘1’ and no successors, but the Late Finish is derived from the latest finish of the schedule. Therefor the Early Dates are 1 and 5 and the Late Dates are 10 and 6 giving a Total Float of 5.
Option 3: The SF link operates in the reverse direction and therefore A follows B and the ES and LS of A is ‘11’ therefore Total Float is 0
Option 4: The calculation option shown above where the forward pass and back pass are calculated using different methods.
The way I believe SF link calculations work correctly is set out on page 11 of our paper ‘Basic CPM Calculations’ downloadable from: http://www.mosaicprojects.com.au/PDF/Schedule_Calculations.pdf Based on this, my view is the link operates from A to B therefore Option 1 applies in the absence of any instruction to allow the schedule to ‘float to the longest path’. The consequence of this would be for B to have a negative float of 10 days.
However, I have a nasty suspicion though the question setter is looking for Option 4 as the answer. What do you think the correct calculation should be and importantly why?
Dec. 11th: Thanks for the discussion below and comments via email.
Based on the problem as presented and a careful review of the comments, I believe the correct answer is:
If the network was properly constructed with a finish milestone and both activities linked to the milestone, the link would still be redundant, but the Float on A would be 5 as per Paul Giammalvo’s example below (the same result would occur if the scheduling option was set to float all ‘ends’ to the latest EF (float-to-longest-path) .
Note: Paul uses a different positioning convention for Early and Late dates.