After Model #4 -Target Behaviors Definition , you have a prioritized list of behaviors that you need people to perform to get something back out of your product. This list ensures that everyone involved receives some value (even if it's just a perception of value).

From this point onward, you could spend as much time refining a feature that will get users to do it. But beware, this is one of the largest pit traps that lay in your way to launch. You could end up spending valuable time over-defining a product. On the other hand, it could be the case where you cannot tackle the challenges before a particular window of opportunity or that the project is so challenging that you may be better off walking away from it. Scoping is key. This section deals with how to get past this stage

Software developers have found a system using the Fibonacci sequence (that has been battle-tested) to estimate the complexity of proposed tasks for development. For those that are not into math, the Fibonacci sequence is the series of numbers that results from adding the last two numbers and goes like 1, 1, 2, 3, 5, 8, 13... And, believe it or not, this method is rooted in behavioral sciences.

The trick is to find the most straightforward task in the lot or think of a dummy task that is the smallest chunk of work you could complete. Then, assign that task a 1 in complexity. Then compare all other challenges in terms of complexity to this base task and with each other. Is something more complex or roughly equal? But, remember, instead of using a continuous scale (for example, 1 to 10), use the Fibonacci numbers.

When estimating task complexity using numbers from 1 to 10, or 1 to 100 is easy to argue if something is a four or a five. They both fall into the "medium" complexity range. However, the Fibonacci sequence, which grows exponentially, forces you to decide if something is a 3, a 5, or an 8. We are still selecting from a scale, where each number has a given position in the list, but it feels different.

The Weber Fechner law explains this phenomenon:

"the minimum increase of stimulus which will produce a perceptible increase of sensation is proportional to the pre-existent stimulus."

Of course, this law has larger implications on perception and can help design a better product. But in this case, it means that the difference between 3 and 2 feels more significant than the one between 9 and a 10. Moreover, that decrease follows a logarithmic shape (I'm again talking to those into math), so using an exponential scale can help us battle this bias.

The Model

Go over all the target behaviors and estimate how complex it would be to find a suitable solution to get people to perform it. For this, Consider your knowledge of this particular actor and if you have examples of other products that manage to do it.

You now have a prioritized list of behaviors to provoke on your users, and you have estimated how complex it will be to design them. In Model #6 -Target Behavior Funnel , we will start working on the actual design, following the priority order we defined before, but keep in mind your complexity estimation! It will help you come to terms with a good enough solution and moving forward to the next. You can also use this to know how long it takes you to tackle each complexity value and estimate how far you are in your design process.