Sorry for giving you a direct link requiring a password to the AACE Recommended Practice on Developing Activities.  You need to go through the identification page first at http://www.aacei.org/resources/rp/.  You do not need to be a member and there is no cost, but apparently they want you to identify yourself before downloading the Recommended Practices.  I hope that this helps.

If you are scheduling an open hart surgery procedure suggest not using activities shorter than 9 days, the patient will die and will never sue you and you still will comply with the 10 days duration requirement.

The scale is relevant to the activity duration, even in construction jobs. What if the job is a two week job, you will need to schedule by the hour and maybe on the field manually adjust the schedule. An example can be as simple as the replacement of a double headed substation on an industrial complex.

I have serious problems with protocols that assume fixed durations without any reference to the duration scale of the job. At home we have contract writers that blindly copy those protocols without a real understanding of what they mean.

Determining progress is relevant but the activity shall be split if the rate of progress is variable. I doubt the resource allocation algorithms consider nonlinear resource distribution, perhaps the maximum, this will result in erroneous and inefficient models. There are many factors that might limit the duration of an activity but the main question shall be, is the activity as defined is compatible with the model. I do not buy the P6 idea on activity steps, is against good scheduling practice that in combination of predefined variable resource usage is wrong.

This is a reference from a communication with Vladimir:

Spider does not permit entering predefined variable resource usage. We think that variable usage shall be defined by the schedule. Predefined means that activity is complex and actually consists of several sequential activities. We suggest certain rules for defining project activities. One and main of them – activity is done by the certain crew from the beginning to the end. And variability may be caused only by the requirements for the same resources on other activities.

There is an option to define material consumption as linear, at the beginning, at the end, or combination. For materials it is logical.

To my understanding it says it a lot, valid but just one of many factors to take into account when defining activities. You shall find a way to get Ron Winter's reference as I believe it shall provide you with other factors to consider.

Best Regards,

Rafael

Activity durations should be no longer than one can properly estimate and control.  This means that the Foreman or Task Supervisor should be able to estimate the remaining work duration at any point in the activity’s active existence.  In addition, most schedulers would say that the activity should not be so long as to be active over more than two reporting cycles.  In the case of monthly reporting cycles, this means that activities should not be longer than 22 working days and probably not longer than 20 working days.

AACE Recommended Practice 23R-02, “Identification of Activities” http://www.aacei.org/aaceonly/rps/23R-02.pdfis a very thorough review of the subject.  The document is free to download.  I recommend that you review this document.  Good luck!