Industrial Engineering and Production Management
The reasons which are basically responsible for the formation of a queue should be that

The average service rate Hess than the average arrival rate
Output rate is linearly proportional to input
Output rate is constant and the input varies in a random manner
All of these

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Industrial Engineering and Production Management
The assumption in PERT is

A project will always be behind schedule, if left uncorrected
Activities are fixed and can't be changed
A project can be subdivided into a set of predictable, independent activities
Cost of project will always be more than the estimated cost, if no timely corrections are taken

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