Saturday, May 23, 2015    
   You are here : PM Topics  >  Time Mgmt


    Michael Altshuler

Sigma Values


+/- 1 sigma 68.26% (68.3)
+/- 2 sigma 95.46% (95.5)
+/- 3 sigma 99.73% (99.7)
+/- 6 sigma 99.99% (99.99)



Variances and standard deviations are a knowledge area helpful in determining probabilities for estimates and occurances. The standard deviation of a probability distribution is a measure of the spread of its values.


If you are studying for the PMP exam, you will benefit from being able to calculate the standard deviation when given two deviations. Standard deviation is the square root of the variance. It is denoted with the letter σ (lower case sigma).


For more information, read this article:

Project Time Management

Project time management is so much more than just personal time management. It includes learning how to plan the process flow of the project, calculate the critical path, processes for deriving good estimates, learning schedule optimization techniques, protecting your project from time wasting activities and interuptions, and you'll find a long list of project management terminology related to the science of project time management.

Time management submenu:

Network diagrams

Calculating the math of the network diagram is a beginner project management skill. See this little 8-minute video on how that math calculation works. (Go to video now.)

Schedule Compression Techniques

Fast tracking: Changing the flow of the network diagram so that you are doing some work in parallel that you had previously planned on doing sequentially. This alters both the network diagram paths and possibly also the critical path.

Crashing: Adding resources (more equipment, people, or vendors) to the project activities to get them done faster than originally estimated. These keeps the sequence in the network diagram, but may alter the critical path.


We consider using Z-scores an advanced scheduling formula that most project managers are probably not expected to know by heart. However, it's good to know where to find this formula when you need it...


The Z-Score is a calculation that helps you answer "Given a desired completion date, what is the probability that this completion date is reachable?"


Statistically the Z-score is the number of standard deviations from the mean a particular value is.  For project management purposes, the value of Z is the number of standard deviations that the desired project due date is from the expected completion time.


To determine the Z-Score you would take the proposed project completion time (T), subtract the expected project completion time (TE) from it, and divide it by the project standard deviation (σ):    Z = (T-TE)/σ

Once you determine the Z-Score, you can translate the z-score into the probability percentage by using a Z-Score table such as the one at


This is the step-by-step process:

Step 1: Calculate the expected completion time (TE) using PERT

TE = (O+4M+P)/6

Step 2: Calculate the variance of the project by calculating the variance of each task

Task Variance = [(P-O)/6]2

Step 3: Calculate the standard deviation

Standard Deviation =  Task Variance1 + Task Variance2 + Task Variance3… .


Step 4: Calculate the Z-Score

To calculate the Z-Score use the equation:

Z = (T-TE)/σ

The Z-Score (z) is the differnce between the desired completion time and the project's expected time divided by the standard deviation for the project.

Step 5: Calculate the probability of success now that you have figured out the Z-Score.

Translate that score (sigma value) into an actual percentage. This translation is done using a Z-Score table. Note that a Z-Score table is not calculated by the project manager—it is a pre-existing table that statisticians have already calculated for use with Z-Scores. The Z-Score contains the same values regardless of the application or industry.

Critical Chain

Read a good description of critical chain here. But when comparing this to normal project scheduling remember that critical chain eliminates the multi-tasking. It buffers all of the slack and contingency into a project pool, instead of building it in along the scheduling.

Types of Reserves





Types of Schedule Dependencies
  1. A start-to-finish (SF) relationship between two activities implies that the completion of successor is dependent on the initiation of its predecessor.
  2. A finish-to-start (FS) relationship between two activities implies that the initiation of successor is dependent on the completion of predecessor. This is the most common dependency used in network diagrams.
  3. A finish-to-finish (FF) relationship between two activities implies that the completion of successor is dependent on the completion of predecessor.
  4. A start-to-start (SS) relationship implies that the initiation of successor is dependent on the initiation of predecessor.
How to Encourage Your Team to do Timesheets

This paraphrases tips from a newsletter article published by  on 3/19/2013. 


1: Explain why you need the information: Team members are more likely to do their timesheets if they know what you are doing with the information. Explain how it helps you manage the project more effectively. The fact is that it provides really useful information to validate the estimates and it ensures that everyone is working on the right things at the right times. You can adapt the project schedule depending on how quickly tasks are being done.


2: Make it easy: One of the reasons why people don't complete timesheets is because they are difficult to fill in and time consuming. Do whatever you can to simplify the process, including looking at tool upgrades.


3: Link the data to the project plan: Try to get the timesheets to post directly to the project plan so that it feeds project reports real time.

4: Review your estimates: Analyze the variance from estimates information and help keep the team members informed so that they can become better estimators and your planning team can revise the plan and reforecast based on these variances.

Lead and Lag

What's the difference between lead and lag? They both go in the same "lag" data box in MS Project!


Lead time is reflected as a negative number on a line in a network diagram.  Lead allows for an acceleration of a successor activity. 


Lag time is reflected as a positive number on a network diagram. It is nonworking time that gets added to a schedule as time that must pass between dependent tasks.


Free Float and Total Float

Free slack is the amount of time an activity can be delayed without delaying the early start of any immediately following activities. Free slack is only about the subsequent tasks - not the overall path slack.


Total slack is how much a task can slip before it delays the project (i.e.: effects the critical path).

Scheduling Challenge!

Scheduling Challenge Project Management Training Game

This team-based exercise teaches participants how to calculate and analyze the project's critical path.


Activity timeframe: 60 - 90 minutes.


Materials include a simulation project with PERT activities preprinted. Activity predecessor relationships, activity costs, and a schedule crashing table.


After analyzing the project situation the teams make schedule-related decisions. From this point forward, they will know how to determine how to make the best decisions on what project activities are the best decisions for shortening.


Kit contents:

  • Instructor PowerPoint including speaker notes
  • Participant materials including the following:
    1. Crash table
    2. Base project information
    3. Base Cost line chart
    4. Preprinted Activity boxes for 3 teams.
    5. Milestones
    6. Red critical path marker
    7. Calculator

The instructor CD includes the file to print more of these if you would like. Also, additional sets can be ordered.

See the training game in the store.


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