Do you understand the risk you are running when two events come to a merging point in your schedule?
 There's a series of tasks running along on one path, call it "A"
 There's another series of tasks, not dependent on "A", running along on path "B"
 But, all the events set to begin on path "C" can't begin until everything on paths "A" and "B" finish.
In effect, the completion of everything along "A" and "B" gates, or controls, the beginning of "C".
So, where is the hazard?
The hazard is that "C" will be late starting if either "A" or "B" are late. Actually, that doesn't sound like such a big deal, so what's the problem here?
It's all in the probabilities. Consider this example:
 "A" probably late 1 chance in 4 [written as: 1/4], and
 "B" probably late 3 chances in 10 [written as : 3/10].
 In 40 chances, we expect "A" to to be late 10 times (1 chance in 4, 10 chances in 40), but ontime 30 times. Of course, "C" will be late those 10 times that "A" is late.
 But when "A" is ontime, 30 chances (out of 40), the performance of "B" determines the performance of "C" ("B" late makes "C" late).
 In 30 chances we expect "B" to be late 9 times (3 chances in 10, 9 chances in 30).
But if late 9 times, then "B" is ontime 21 times  Consequently: "C" is expected to start ontime 21 of 40 trials, or just over 50% (about 1/2)
 But, that means "C" is expected to be late almost half the time  10 late starts from the effects of Path A and 9 more from Path B. Altogether, that's 19 late starts out of 40  a serious performance degradation from either that of "A" [25% late, 10 out of 40] or "B" [30% late, 12 out of 40]
(*) the common denominator of 1/4 and 3/10 is 40
We can show all this with this mapping chart:

Path A 
Path B 
Path C 
Probably late 
1/4 
3/10 
1 – 21/40 
Probably ontime 
3/4 
7/10 
21/40 
Independence simplifies:
Notice that along the bottom row, Path C is just the multiplication of Path A and Path B probabilities
Along the top row, the probabilities in all cases are just 1 bottom row, cell by cell. [the number 1 represents all possibilities]
These calculations are only valid if Path A is in every way independent of Path B. If not, then there is crosstalk between paths that will degrade the calculations.
But in a project, what does independence mean?
 No shared resources that could cause conflicts
 No shared lessonslearned after the tasks on Path A or B begin
 No changes in "A" because of what is happening in "B"
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