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Thursday, 25 January 2007 19:16

Activity Duration Estimating

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Duration includes the actual amount of time worked on an activity plus the elapsed time.

Effort is the number of workdays or work hours required to complete a task. Effort does not normally equal duration. People doing the work should help create estimates, and the PM should review them. Duration estimating is assessing the number of work periods (hours, days, weeks,) likely to be needed to complete each activity. Duration estimates always include some indication of the range of possible results, for example, 2 weeks + or – 2 days or 85% probability that the activity will take less than 3 weeks. Activity Duration Estimating:

 

  • Perform reality checks on schedules.
  • Allow for contingencies.
  • Don’t plan for everyone to work at 100 percent capacity all the time.
  • Hold progress meetings with stakeholders and be clear and honest in communicating schedule issues.
  • Goals are to know the status of the schedule, influence factors that cause schedule changes, determine that the schedule has changed, and manage changes when they occur.
  • Tools and techniques include:
  • Progress reports.
  • A schedule change control system.
  • Project management software, including schedule comparison charts, such as the tracking Gantt chart.
  • Variance analyses, such as analyzing float or slack.
  • Performance management, such as earned value


Calculating Duration

For example, if you have a 10-hour activity and you have two resources assigned to that activity, what is the elapsed time assuming 100% productivity of each of the resources? The answer to this type of question is very dependant on the context of the question (the way it is written). For this question, the answer is two resources, 10 hours, 100% productivity. You can say it is going to take 5 elapsed hours to get that task accomplished because it is going to have two people working on it, 5 hours apiece, to accomplish 10 hours’ worth of effort for that given activity.

However, let’s look at it from a different perspective. Suppose they are only available 50% of their time. If they are only available 50% of their time on a given day, then it is going to take you a grand total of 10 elapsed hours. Why? They are each only giving 50% of their time. Thus, it is going to take a full 10 hours to get that same activity accomplished. What if their productivity is lower? Suppose they are only 50% productive and they only afford you 50% of their availability. Well, if they are only 50% productive, that means they are half  as productive as the people who normally do this work, and it also means that you are only getting half of their time to begin with, so instead, your elapsed time is going to creep out to 20 hours. You need to be prepared to take all those parameters into account when it comes to estimating durations. You need to be thinking through them.

 

Analogous Estimates

Analogous estimating means that you use the actual time frame from a pervious, similar project as the basis for estimating the time frame for the current project. The underlying premise is that this project is analogous to that project. Because we know how long that project took, we can estimate how long this project will take. Analogous estimating is used when little information is known about the current project or when two projects appear similar. Of course, if they appear similar but, in fact, are not, the estimate is inaccurate. Analogous estimation is good for ball-park figures but not for precise estimations of a project or timeline development


Parametric Estimating

Parametric modeling involves using variables from the project description in a mathematical formula to develop an estimate. For example, if you know the number of units that will be involved and how long it takes to create one unit, you can estimate the amount of time it takes to create 1000 units. If it takes your team 10 minutes a unit and you need 1000 units then it will take 10,000 minutes, which is 167 hours or a little over four weeks for development. The formula is simple: the number of units multiplied by the amount of time each unit takes to create equals the total amount of effort needed for development.

 

Three Points estimating

Three-Point Estimating is a quantitative analysis technique that assigns numerical values to define a range of possible out-turns so that Risk Analysis may be carried out to better inform decisions. Three-Points Estimating may also be referred to as PERT (Program Evaluation and Review Technique). It makes sense in the real world that you do not really know how long a particular activity will take, specially talking about certain activities such as research and development. In this case, we can look at the project completion time in a probabilistic fashion and for each activity we can define:

  • Optimistic time estimate: an estimate of the minimum time an activity will require.
  • Most likely time estimate: an estimate of the normal time an activity will require.
  • Pessimistic time estimate: an estimate of the maximum time an activity will require.

Project managers can use three-point estimating to gain a greater degree of control over how the end value is calculated. The end value is the weighted average of three estimates. To do three-point estimating, in its simplest form for a particular task or activity, ask the resource for their best, most likely and worst estimates. Add the Pessimistic estimate to four times the most likely, then add the optimistic and divide by six. This gives you your estimate, which is a slightly more balanced view of how long the task or activity is likely to take. The calculation is (P + 4M +O / 6).

 

Precedence Diagramming Method (PDM)

time-pdm.jpg

PDM is also referred to as Activity-on-Node (AON). The PDM puts the activities in boxes, called nodes, and connects the boxes with arrows. The arrows represent the relationship and the dependencies of the work packages. Example:

PDM includes four types of dependencies or precedence relationships:

  • Finish-to-Start
  • Finish-to-Finish
  • Start-to-Start
  • Start-to-Finish

pdm_precedence_relationships.jpg

 

Arrow Diagramming Method

The arrow diagramming method (ADM) is similar to the PDM, except that all dependencies Activities and durations are generally depicted on the arrows. For this reason, the ADM diagram is also called the Arrow-on-Arrow (AOA) diagram. Example:

time-arrow_diagram.jpg

ADM uses only finish-to-start dependencies and can require the use of “dummy” relationships called dummy activities, which are shown as dashed lines, to define all logical relationships correctly. Since dummy activities are not actual schedule activities (they have no work content), they are given a zero value duration for schedule network analysis purposes.

 

Dependencies

  • Mandatory dependencies: often involve physical or technological limitations of the work; for example, a prototype must be built before it can be tested   
  • Discretionary dependencies: Also referred to as preferred logic, preferential logic or soft logic and are based on knowledge of best practices within a particular application area.
  • External dependencies: any input that is needed from another project or source outside the project team.

 

Float, Leads & Lags 

  • Free Float - The amount of time an activity can be delayed without delaying the next scheduled activity's start date
  • Total Float - The amount of time an activity can be delayed without delaying the project's finish date
  • Lead - allows an acceleration of the successor activity.
  • Lag - directs a delay in the successor activity


Note: Float and Slack mean the same thing.

 

Read 14643 times Last modified on Friday, 11 December 2009 01:27

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