Efectos sobre el EEG durante la fase REM

NOMENCLATURE

Professional project scheduling is built on the fundamentals of the critical path method, or critical path management as it is has evolved to be known as in the industry. Critical path method refers to identifying bottlenecks in the production process and then building a production timeline through them, using the total duration of those activities that will take the longest to com- plete as the basic timescale. Critical path management is the methodology for managing those timeline paths efficiently, by manipulating time management of the interlinkage of activities durations and contingency workarounds. It is a scheduling system that allows the project scheduler to achieve improved time control over a project’s production phases. Additionally, an audit trail and cost tracking can be incorporated into the system to provide a reasonably accurate estimate of timeframes required for those or similar activities in fu- ture projects.

CPM typically works best in straight-line, time-scaled productions in any business where the timelines can be estimated with a fair amount of cer- tainty. The modern network scheduling software programs are essentially a combination of CPM and S-charts. Critical path management is the most accurate computerized system of network scheduling yet developed. It allows production managers, developers, owners, and prime contractors to achieve control in the following critical areas of project scheduling:

They can figure out where they stand right now in the project. They can determine where to expect production bottlenecks.

They can decide what to do next.

They can begin changes today to keep the project on track.

In the construction industry, repeat business does not just come from simply building something well. The companies that get repeat business do so because they manage projects in ways that protect their clients. CPM sched- uling tells the owner where he stands, not only financially but also in terms of issues, problems, and resources on the project. Ultimately, that’s the biggest concern to most owners. CPM has proven itself by adding more bottom line to owners’ investments. In larger projects, such as multiresidential, commer- cial, industrial, and public works, CPM is mandated by contract.

All production activities will affect each other in either direct or indirect linkage. CPM fundamentals require that a critical activity must be finished before the succeeding critical activity can start. Noncritical activities and those of similar trades, such as roughing in the plumbing and electrical work, can proceed simultaneously. One of the objectives in critical path manage- ment is to find the order in which the phases of critical activities must be completed. Those designated as critical activities begin by being assigned to the production activities that will take the longest to complete and are crucial to the production timeline. By tracking these activities, attention is automati- cally drawn also to those areas where it is most essential to avoid production delay.

When bottlenecks occur in the schedule, those activities and their sub- activities on the critical path must be handled first, while those delays off the critical path can be addressed secondarily. This is especially true if a particular activity has zero or negative float. Delays along the critical path begin affect- ing total project duration. Critical paths may change as production bottle- necks are broken, as critical subactivities are changed, or when new problems surface.

CPM is currently is the only production scheduling system that covers all the phases of a project and allows the project schedule to manipulate time- frames during and around the activities that will take the longest time to com- plete, then selecting the best ways to expedite or work around the phases of activities for contingency planning. The underlying logic here is that con- tingency planning reduces risk. When this time-scale interlinking of produc- tion activities is combined with a software tracking system, the scheduler can then provide computerized audit trails and cost tracking reports, as well as trends analysis, which is next quarter’s forecast of the current numbers logi- cal outcome. These are crucial functions for protecting your client’s vested interests in the project and are major selling points of your services as a pro- fessional project scheduler.

CPM can be thought of as the path of least float, based on the relation- ship between sequential activities and completion duration of the activities.

Think of float as scheduling time leeway, or slack. In developing a CPM schedule, the critical path is first determined by identifying all activities with zero float time. Any activity is considered critical if its completion delay will cause total project completion delay. Critical path activities with zero float time must begin when scheduled, or the total project completion date will be pushed back. The arrow diagramming method (ADM) and the precedence diagramming method (PDM) are the two basic zero-float-time scheduling sys- tems from which CPM has evolved.

One of the cost-effective benefits achieved by using CPM scheduling is the analytical consideration of activities duration and job logic sequencing of each activity in the initial stages of the project as the CPM schedule is being developed. An arrow diagram can describe an example of such a CPM net- work diagram. The tail end of the arrow indicates the start of an activity, and the head end represents the completion of that activity. Using a graphic ar- row to indicate paths on a flowchart, each activity (or arrow) will have a start, stop, and duration. When two or more arrows or activities meet, the inter- section is called an event. Activities are begun and completed at events, and succeeding activities in their production phases move forward from one event to another event. The events are assigned numbers, which the computer uses to change event sequences or durations as the CPM program is monitored and recycled during the production period. The various activities and events in the CPM schedule, making up the network diagram, are interlinked by inter- dependence and the project’s timeline.

A CPM schedule shows the interdependence of one activity on a preced- ing or succeeding activity, much like a shadow. For instance, if the excava- tion for the building’s footings is scheduled to commence 10 days before forming activity commences and 16 days before placing of the concrete event commences, any delays due to late delivery of materials or inclement weather will cause subsequent activities to be delayed, pushing back the contractual completion date. Typically, in modern commercial projects, a structural-steel framing system will be erected on the building’s foundation. So if, in our example, the earthwork excavation delayed the forming and pouring of the footings, the delay would also push back the entire phase of activities in- volving placement of the steel and iron workers. The CPM schedule will also show what effect inclement weather delays will have on foundation work, completion leading to the structural-steel starting events and finishing events, and the relations of these changes to all other construction phases. If there is zero float time on the critical path, the project scheduler using CPM can decide whether the concrete foundation work is to be accelerated or the steel erection can be delayed.

Showing interdependence of one task, or work item, on another is the major difference between bar charts and CPM scheduling methods. Whereas the standard bar chart may show a continuous line of activity for a particular

trade, which starts at one point and continues uninterrupted to another point, the CPM chart draws attention to the specific starting and ending dates (events) for each major portion of a construction component. Critical path management will also show the interdependence of each activity and the effect of one event finishing late or early on the starting event of the succeeding construction activity.

CPM scheduling has its disadvantages also. It will increase the total contract price. Such schedules are expensive to create and maintain. In ad- dition, a professional CPM scheduler usually must be hired to develop and manage the schedule. The project team, or at least those who are responsible for production, must be trained in CPM. Finally, small- to medium-sized con- tracting companies typically believe such a sophisticated schedule is unneces- sary and a hindrance to work. In such a case, the subcontractors developing and maintaining a CPM schedule may be haphazard at best. However, re- quiring the contractors to construct and maintain a CPM schedule has at least three advantages. First, it requires the contractors to work more effi- ciently. Second, it gives the owner an instantaneous summary of the actual progress to date of the project. Third, from a litigation standpoint, requiring the contractors to maintain a CPM schedule helps prove or disprove financial claims and change orders.

CPM project scheduling starts with the preparation of a network diagram displaying all project planning and construction activities required for the project’s completion. The primary work items needed to complete each activity must be identified, located, and lined up. The scheduler then determines which tasks must be completed before each following work item can be started. Once the overall length of time is determined by adding all the phases that will take the longest, the job logic order in which these activities must be completed is established. Finally a network diagram, which is an activities flowchart, is worked out.

Some project activities precede others on a straight-line basis and cannot start until a prior activity has been completed. Other activities can start prior to the completion of a preceding activity, while other activities are performed simultaneously or concurrently with others. Those activities whose durations are crucial to the overall project completion date are considered to be on the critical path. These are the basic factors necessary to construct a CPM network.

Next comes the scheduling phase, in which an estimate of the time required to accomplish each of the activities is developed. This is really an educated‘‘guesstimate,’’ so we have ways of dealing with inside and outside figures for factoring a time cushion, which we call float. The third stage involves installing these estimates into the activities network diagram and, finally, in the fourth stage, computations are made of data and critical paths

of activities to provide the time-scaled network of the project. CPM is tra- ditionally explained as this sequential order for illustration. However, in real- world usage this is rarely the standard procedure. And that’s the crux of the matter right there. There is no standard procedure. Because each project is unique, no single formula can factor in all the intangibles. The computers can do instantaneous, error-free computations, but it will always take an ana- lytical mind using experienced critical thinking to program and run those numbers profitably.

For example, in the time-compressed business world of today, these four basic planning steps often are completed simultaneously. Those trained in CPM use programs that‘‘wash through’’ this data instantly to each stage, and change datum in all related stages if one factor is changed, using‘‘what- if ?’’ scenarios. For the sake of instruction, though, we will use the assumption that they are each treated separately and in the order listed above.

Timeline computations are just simple addition and subtraction. The task of time-scaled computation is usually just as simple and easy. However, managing the timelines of the many tasks and activities involved in a modern project quickly becomes an enormous challenge. You will see, as your career progresses, that there are days when your CPM network timeline feels like a nervous mustang ready to leap out of control. Accordingly, most schedulers use computers to manage their CPM timelines. And with the use of dedicated software programs, the timeline can be updated regularly without redoing the entire network diagram. Complex time variables can be‘‘best line linked.’’ Forecasting by use of trends analysis shows the probability of various outcomes. Each activity can be updated regularly for accurate data factoring. Typically, this is done in daily updates. This frequency serves two purposes. First, it guarantees timely accuracy in your data and pries open the largest window of opportunity necessary to make changes and work- arounds before problems start costing money. Second, it produces a superior kind of detail orientation, combined with fast results, that tend to prevent budget cost overruns and give the astute project scheduler a definite advan- tage over the competition. This is the type of professionalism that produces profit and builds the reputation that you are worth a larger paycheck. Your competition consists of scheduling management companies that specialize in all levels of network scheduling, for contractors, production factories, architects, and owners.

The complex scheduling problems that occur in commercial CPM can be worked out rapidly by using program logic in a dedicated software network system, and time management becomes a viable tool for working out options, workarounds, and solutions. A programming task that normally takes hours to complete, if done by hand, can be accomplished in seconds by computer programs incorporating program logic. This rapid access to data is one of the

key reasons for the successes of CPM. This speed of program logic allows variables to be worked out or planned around before they cost money.

TERMINOLOGY

To better understand critical path management, you first must know the lingo of CPM, along with the terms’ meanings in network diagram scheduling. Here are the principal terms and definitions of procedures of network scheduling, in order of their precedence:

Task. A task is an individual unit of work that may combine with other tasks to complete an activity or be independent work items. Tasks can be thought of as the separate work unit items that collectively need to be done to finish each activity sufficiently enough to start the next activity.

Activity. Activities are any single identifiable work step in the total project’s production. Groups of tasks, combined to finish a job item, are activities. Once the activities have been identified, their sequential task logic is established. Then begins outlining the project graphically into a network diagram. In a velocity diagram, the symbol for an activity is an arrow. The arrow connotes linear timeline movement from left to right, start to finish. The continuity indicator is the arrow from one activity to another.

Activity number. Number assigned to each activity. These numbers should be sequential. The computer will not be able to establish job logic if activity numbers are nonsequential.

Activity list. List of work items for a project; also the work breakdown structure.

Activity duration. Elapsed time to perform an activity, start to finish. Arrow diagram. CPM network diagramming method using arrows to show activities interrelationship and the flow of job logic.

Event. An event is the exact day at which an activity is just starting or finishing. Network program logic applying to all events is that all activities leading into an event can be started at that time. An activity is always pre- ceded by an event and followed by a sequential event. Thus, an activity al- ways has both a starting event and a finishing event. Theoretically, that finishing event is the starting event of the next activity. However, one must always account for the natural constraints of the real world and provide a timing leeway cushion allowance as problems in actual production will require the ability of contingency planning for tasks and/or activities that may get bottlenecked along the way. Proper use of float is the method of contingency planning here, which is known by the term workaround.

Event diagram. The most common event system uses circles at the ends of each activity arrow. These circles are placed at the junction of the arrows

and they represent the event, or the moment of time at which an activity is just starting or finishing. Important events that mark the completion of a phase, such as a foundation final, are called milestone events for they serve as benchmarks to your schedule’s progress versus run time.

Milestone. Date on the schedule predetermined for a phase or important occurrence is scheduled to take place.

Early start date. Earliest date an activity can start. Late start date. Latest date an activity can start.

Early finish date. Earliest date an activity will be completed without float.

Late finish date. Latest date an activity will be completed without negative float.

I–J number. The letter i designated symbol for the tail of an arrow (start of an activity), and the letter j designated symbol of the head of an arrow (finish of the activity).

Float. Measure of available scheduling leeway time on any activity’s completion.

Free float. Time that activity finish can delay event without affecting the succeeding activity’s start event.

Total float. Measure of available spare time or scheduling leeway available on all activities’ completion. Total sum of all free float.

Negative float. Time a critical activity is late meeting its finish event. Phases. Phases are groups of activities that will happen in a logical order, precede or succeed one another or happen simultaneously. As tasks are the micro-units, phases are the project’s macro-units. Phases are arranged in divisions with sequential velocity management. Phases are supposed to flow into each other smoothly, on time, with no problems. If problems are anticipated, contingency planning in the form of workarounds needs to be established ahead of time.

Constraint. A constraint is a potential real-world limitation, or a tactic of the scheduler in creating a float window during monthly recycling of the schedule, that can delay the starts of activities or tasks. Constraints, known as a dummy activity on network schedules, are shown as dashed lines with zero elapsed time. Constraints are negative factors that balance the calculations of job logic.

Job logic. This is the sequential relationship between activities, identi- fied and defined during preschedule planning. These relationships consist of the necessary time and sequential order of construction operations through- out the project.

Logic diagram. Arrow diagram of complete project network schedule, or a cross section of an area of production.

Velocity diagram. Velocity diagram means a straight-line, time-sched- uled flowchart. The purpose of the velocity diagram is to determine the most efficient paths to joining the activities in total operational network scope. Traditionally, the arrows in a velocity network diagram represent the activity itself, not the direction of movement. The angle of slope and arrow’s length are not factors in the scheduling, simply the designer’s choice. Each arrow in the velocity network diagram represents an activity, identified by its activity