I'll take this time to explain a little more about the cone of uncertainty for those who are not familiar with it, or who would like to see a more practical example of what it is. To do so, let's consider 10 flips of a coin as the example. There are 2 to the power of 10 possible combinations of 10 head or tale results.

Before rolling the die a first time, I want you to guess what the final total may be. How many heads do you think you'll get?

Well, if you think about all the combination (0 heads, 1 head, 2 head...) and thus build a histogram of all results, you get this:

number of heads when flipping a coin 10 times - University of North Carolina (via Google images) |

#### 1st Flip

When you flip the coin the first time, it comes up say, tails. This does two crucial things:- It gives you an
*actual*result to work with, so you now have 9 uncertain results and 1 actual result. - Now that you have flipped a tail, you
*cannot*get 10 heads. Given that in the above histogram which applies all the time, there is only one scenario, that scenario is now out! The best you can hope for is 9 heads, given you've flipped 1 tail.

#### 2nd Flip

Flipping the coin the second time, it comes up say, heads. This also does two crucial things:- It gives you an
*actual*result to work with, so you now have 8 uncertain results and 2 actual results. - Now that you have flipped a head, you
*cannot*get 0 heads, because you have at least 1. Given that in the above histogram which applies all the time, there is only one scenario with 0 heads, that scenario is now out! Your rage is now 1 head to 9 heads.

Put this in the table and flip again. Follow the rule that if you flip a head, you increment the minimum by one, otherwise you have flipped a tail so decrement the maximum heads by one (because you now don't have enough flips to get the previous maximum).

### <<Fast Forward>>

#### 10-Flips

*10-flips completed*

So we've completed the whole 10 flips, incrementing the minimum if we get a head and decrementing the maximum if we get a tail. Surprise surprise, by the end, we have two ends that meet in the middle (which is correct, because by that point, we have 10 actual results and thus, no uncertainty at all). You can double check this by counting the number of heads you got, which is 4 in this case, against the meeting point of the maximum and minimum, which is 4. If you don't, then you've banjaxed your counting, so you might want to ask a 3 or 4 year old for help next time.

### Making the Cone

From this table, we simply have to plot the flip number against the minimum and maximum number of heads. So let's do that. I've also included the trend lines, in black, which show the trajectory of the minimum and maximum numbers. The gap in the middle is the level of uncertainty or variance:

cone of uncertainty |

Let's recap what happened. At the beginning, we had no idea where we were going to end up [with how many heads], aside from the range of 0 to 10 heads. As we progressed, we reduced the size of the range of possible 'options' or heads we could get and by the end, we were where we were.

Map this to typical IT projects. At the beginning, we have no idea where we're going to finish. As we progress and choices are made (which honestly do sometimes seem random), we reduce the total number of potential options that we have (which isn't always a bad thing, especially if we discount the highest waste or risk options) and eventually, we come to rest somewhere. Also, despite everything, we always know where we're are starting. We're starting 'here'. The end of the last cone (or part thereof).

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