CASE-STUDY: PERT TASK DEPENDENCY
A project is set of activities geared towards a specific purpose. These activities can be explained by a graph or a flow chart. For a graphic presentation, there are mainly two formats: Arrows and Boxes. The later is known as AON (Activity-On-Node) which contain many additional features like Floats and Task-Dependency.
Floats have already been covered in a previous hub. Here I would explain Task-Dependency.
WHAT IS A TASK DEPENDENCY
Task Dependency is a linkage of a particular task with others called Predecessors. It is a logical relationship. An activity can start only after a certain activity is completed. Two activities may start together or finish together. End of an activity may depend on the start of another activity. If you are good in counting, you might have already noticed that there are four types of linkages between any two activities. This is summarized below:
- · Finish-to-Start
- · Start to Start
- · Finish to Finish
- · Start-to-Finish
Building rooms is dependent on acquiring land. Construction cannot begin before land transfer is complete. This is a default dependency. If there are two tasks, “Dig Foundation” and “Pour Concrete”, there is no way of pouring concrete unless excavation work is finished.
The dependent task can only begin after the one, it depends upon, starts. For SS relationship, the two activities must be linked but one of them started earlier and the other to follow. In other words, these activities must start one after another with a small time-lag. The purpose would not be served if one starts and the other doesn’t pickup or starts with a long time lag.
Supposing there are two tasks: (i) plastering and (ii) painting of thirty rooms. Now it is not necessary to first plaster each and every room and then start painting them. It would much better to ascertain, how long it takes the plaster to dry up. If it takes only one day, the painting can commence next day side by side. Please note that the two activities are linked. If only plastering is done and left for long, it would again require scraping, sanding and filling before a paint can be applied.
A task cannot be completed until another task is complete. Here again, time lag must not be large else it would look like FS relationship.
Supposing, we have an assignment to recruit as many as drivers we can find. At the same time, we must match an equal number of conductors. Two teams may start interviews but team concerned with drivers must finish its job before other team can complete its job. Obviously, first team would announce the number of drivers they have found suitable for the job. Same number of conductors can now be hired.
A dependent task cannot be completed unless another task it depends upon has started.
Once again, this implies ‘freshness’. We cannot mix paint and leave it to rot. We wait a signal from the user about the exact time it is needed and would finish mixing a little before. This is reverse of FS where we can purchase the land and start building even after 5 years.
We cannot reverse it like looking for a plot of land when our building design has already finalized. We cannot even design a building as it would depend on plot dimensions, road accesses, streets, sub-soil conditions, neighbor-hood structures, wind and sunshine directions.
In FS relationship, mixture of paint depends upon start of painting. Painting can be done any time. It is not that paint-mix is ready and now painting must start. Who order for the paint mix? It should have been done only when needed.
Given below are examples for practical experience in task dependency:
- Machine cannot be installed unless machine-foundation is complete
- Trial runs cannot be started unless machine is fully installed.
- Baking 10 cakes is obvious to decorating which starts after first cake is finished.
- 50 resumes can be studied and interview can be started after first.
- Planning for the construction phase can begin before final design approval but it cannot be finished until design is finally approved.
- Salesmen require back-up staff. Hiring of salesmen and back-up staff can be started at the same time but hiring of salesmen must be completed first in order to ascertain how many support staff would be required.
- In heart operation, heart is stopped and OXYGENATOR is started. It is only switched off, when heart starts functioning.
- When electricity is coming through generator, it must be switched off first before turning to main supply.