Critical paths

A technique widely used by the industry is Critical Path Analysis (CPA or Network Analysis ) which is a method for systematically analysing the schedule of large projects, so that activities within a project can be phased logically, and dependencies identified. All activities are given a duration and the longest route through the network is known as the critical path. 

In the example above the relationship between four activities of different duration is shown. In this case the critical path is indicated by the lowest route (six days), since the last activity cannot start until all the previous activities have been completed. 

Critical Path Analysis—studying general procedures. 

For a given particle of size d, from the point M where the equilibrium line meets the line of zero vertical velocity (see Fig. 13.4), the critical path of the particle may be defined. All particles of this size between points D and G are entrained in the downward stream and are collected. The remaining particles of this size join in the upward-moving stream of fluid and penetrate the cyclone. The point D may be obtained by tracking back the particle trajectory from the point M using the equation of the particle trajectory, which is given by... 

The standard requires that measurements be defined, analyzed, and reported to management at appropriate stages of product realization and that these measurements include quality risk, costs, lead times, critical paths, and others as appropriate. 

The extent to which planned tasks are being completed on time The degree of slippage or slack in the program The critical paths and changes in criticality Lead times and effect of changes on advanced procurement Resource utilization Spend versus budget Estimated spend to completion... 

Most detailed programs for plant-related projects are prepared using simple critical path analysis (CPA) techniques. CPA is a well-known and familiar technique to most engineers and is incorporated into most modern PC-based planning and scheduling systems. Such a system typically has the following features that a plant project manager will find useful ... 

General program discussed. At this stage the main contractor may not have drawn up a bar chart or critical path program ... 

This section is devoted to using critical path method (CPM) as a tool for the scheduling and sequencing of work for shutdowns and project work. 

The critical path is highlighted which indicates the proper or recommended path to follow. 

Confirmed changes or what if scenarios are easy to plug in and a new time line or critical path can be printed. 

Timeliness Time to complete any tasks of project may exceed schedule. By closely monitoring actual progress against schedule along critical path. Look for ways to improve efficiency, attempt to capture time from later steps, authorize overtime if budget permits... 

By the use of the critical path scheduling method, communications with operations and other affected functional groups, the daily work schedule is developed. A shutdown list indicating scheduled start and completion dates is then typed indicating assignments by supervisor. Each work order will be scheduled by central planning. 

Each maintenance supervisor involved in the shutdown should attend and be ready to quickly report the progress of each job as planned and scheduled for that day. In addition, the add-on list should be quickly reviewed daily. The shutdown critical path schedule will be reviewed and updated to track the progress made to date. 

The process of updating the schedule is basically the same as that used in planning the shutdown. It is simply based on new information and deals only with the remaining portion of the shutdown. In some very large shutdowns, it may be desirable to perform time-cost trade-off analysis, resource leveling, etc., each time the shutdown CPM is updated. In many cases, however, the update will involve only the basic forward, backward and slack computations. These calculations will allow for development of a revised schedule. They will also identify the critical path that is likely to change over the course of the shutdown. 

FDA. Innovation or Stagnation Challenge and Opportnnity on the Critical Path to New Medical Products. March 16, 2004. http //www.fda.gov/oc/initiatives/ criticalpath/whitepaper.pd f. 

Can the tire be changed in 470 seconds if only two people are working Let us consider that the father performs every activity on the critical path. This fully occupies his time. One of his children must then be able to perform all the other activities, and reach every node at the same time or before he does. The activities that are not on the critical path and must be done before task I can begin are A, B, F, and G. The total time needed to complete these tasks is 255 seconds. To complete thethe job in 470 seconds, task I must be started 190 seconds (time to complete C, D, E, and H) after beginning the job. Therefore, two persons cannot hope to finish the job in 470 seconds. The reader should try to figure out the shortest time that two people would take to finish the job. Could three persons finish it in 470 seconds ... 

For instance, a company might have a 50-day supply of product available, but be incurring a 250-a-day fine from the Environmental Protection Agency for each day it operates without a scrubber. Under these circumstances, it might be expedient not to crash those activities on the critical path whose slopes are greater than 250 per day. 

Source Cosinuke, W. Critical Path Technique, monograph published by Catalytical Construction... 

Whether the person feels the activity is on the critical path. 

This estimate is used in much the same way the normal time estimate was used in constructing a CPM diagram and determining the critical path. Then an estimate is made of the probability of completing the total job in a certain time period. 

The slack is the difference between the latest time at which an activity must be completed to meet a schedule and the earliest time at which it can be completed. This would be the same as the float if the normal rather than the expected times are used and if the desired completion date is the minimal time required to complete the project. The items with the minimum slack are on the critical path. Thus, by following this procedure the items on the critical path are determined without the use of a diagram. 

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