Can someone give me example for Top-down Approach in term of Estimating Cost and Time

There are so many approaches - often depending upon your industry sector and project/programme context.

Using a mix of analogy, parametric and top down to assess a high level estimate for a conference/seminar; 8 speakers, 2 days for upto 200 guests.

you might split this into venue, transport/logistics, food/beverages, speakers.

Then top down/analogy/parametric - you'd use example costs from similar events to set up approximate values for each sector. For example ...

2 days venue hire at $900/day
2 days delegate package at $30/pp/per day
Transport/logistics for helpers and speakers only - approx $200 each (x volume of people)
Speaker rates say $1200/speaker x 8 speakers.

I'd then wrap a risk plan around that showing margin for change/further scope. This may include number of helpers/volunteers, health and safety surveys etc.

You'd do the same for set up/breakdown time ... and think about capturing recommendations/lessons learned for future.

Always when doing a top down approach, ensure that all key stakeholders recognise the potential inaccuracy of this quick/inaccurate guideline (estimate).

Hope that helps a little?

In top-down estimation approach, the direction of efforts begins from the total project estimate (TOP:entire project estimate) derived from parametric or analogous basis. This total sum is then apportioned to individual efforts required to complete the project on the basis of their contribution.

For instance, for a software upgrade project, an analogous estimate of $4M was available which was adjusted for economic variables (e.g inflation, rise in cost of labour) and adapted for the new project as $5M.

This estimate of $5M could then be distributed over 3 subsidiary project efforts, say (Licensing, Installation & Testing - $2.5M), (Data Migration & Integrity - $1M) and (Documentation & Training - $1.5M).

Each of these efforts is further broken down to lower level and the apportioned estimate (e.g $1.5M for Documentation & Training) to each is also further distributed across the new lower level efforts, subject of course to historical data, experience etc.

And this continues in a downward trend...top-down, to the lowest level (activities)

I hope this makes some sense.

It is very good knowledge. Thank you so much