Ashwin,

You use the technique that makes most sense at any given moment. If your boss asks you for a quick estimate, then you would not be using bottom up because it's not quick. And when the boss asks for a precise estimate with ranges, then you'd use a three point estimate and not analogous. Let's look at the three examples

In the Estimate Activity Resources, we use the Bottom-up Estimating.

Our task here is to estimate "who" or "what" we need to complete a task. So for example: "We need 1 database administrator to work on this task." We will be doing this for every task at the lowest possible level, so it's bottom up.

In the Estimate Activity Durations, we use only Analogous Estimating.

Actually... we use three forms: analogous, parametric and also three-point estimates. And one thing that most people don't realize, is the fact that parametric and three-point estimates are a form of bottom-up estimate. They allow you to make precise estimates at the lowest level. So by extension, we are in fact using a form of bottom-up estimating.

In the Estimate Costs, we use both Bottom-up and the Analogous Estimating.

Actually... we use four forms: analogous, parametric, bottom-up and three point estimates.

It's confusing - I agree. How can you overcome the confusion? Here are three statements that may help you. They may not be 100% perfect, but they help you to remember it for the exam

- In Estimate Activity Resources we only want to do one thing: Determine who does the task. So we only use one technique: bottom-up.
- In Estimate Activity Duration we want to know how long it takes. So we have to be more precise and we use analogous, parametric and three-point. But because we are not interested in aggregating the durations, we don't use bottom-up as an explicit technique. (Aggregation happens in the next process Develop Schedule).
- In Estimate Activity Cost we need to be as precise as possible (because money is important), so we use all four techniques.

Like I said... maybe not a 100% perfect description but it should help to jog the memory.