What is top down estimation?

Quote"The top-down method is also known as the analogous method.
It is used to determine order of magnitude estimates in the initiation phase of the project.
The method uses the actual durations, effort or costs from previous projects as a basis for estimating the effort or costs for the current project.1.Identify a previous project or section of a previous project that is similar to the current project.2.Assess the extent to which the current project is similar to the previous project – the comparison factor (e.g1.5 if the current project is estimated to be50% larger).3.Compute the estimate for the current project based on the actual durations, effort or costs from the previous project and the comparison factor." End Quote