Expected times in weeks, as well as variances, for the major activities of an R&D project are depicted in the following table. Determine the probability that project completion time will be:
a. 50 weeks or less
b. More than 50 weeks

Question

Path	Activity	Mean	Variance
	A	16	.69
Top	B	11	.69
	C	24	.11
	D	5	0
Middle	E	18	.25
	F	26	.11
	D	5	0
Bottom	G	10	.25
	H	14	.36
	I	12	.11

Accepted Answer

Compute the mean and standard deviation for each path:

Path	Expected Time (weeks)	Standard Deviation (weeks)
Top	16 + 11 + 24 = 51	[latex]\sqrt{.69+.69+.11} =1.22[/latex]
Middle	5 + 18 + 26 = 49	[latex]\sqrt{.00+.25+.11} =.60[/latex]
Bottom	5 + 10 + 14 + 12 = 41	[latex]\sqrt{.00+.25+.36+.11} =.85[/latex]

a. Compute the z for each path for the length specified. For any path that has a z of +3.00 or more, treat its probability of completion before the specified time as 1.00. Use

[latex]z=\frac{50-t_{path}}{σ_{path}}[/latex]

The probability that each path will be completed in 50 weeks or less is shown in the corresponding diagram. (Probabilities are from Appendix B, Table B.) The probability that the project will be completed in 50 weeks or less depends on all three paths being completed in that time. Because z for the bottom path is greater than +3.00, it is treated as having a probability of completion in 50 weeks of 100 percent. It is less certain that the other two paths will be completed in that time.
The probability that both will not exceed 50 is the product of their individual probabilities of completion. Thus, .2061(.9525)(1.00) = .1963.
b. The probability that the project will exceed 50 weeks is the complement of this number, which is 1.000 − .1963 = .8037. (Note that it is not the product of the path probabilities.)

Question 17.p.4: Expected times in weeks, as well as variances, for the major...

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