When should a risk be avoided?

You are currently the project manager for a software development project, developing a next generation data-mining tool. Your team is developing a new software product that will help customers better manage large volumes of data, especially for data reporting purposes. Midway, through the project, your engineer decide to add additional reporting functionality to better analyze correlate data. Although this functionality was NOT part of the original project plan, this feature seems like it could be a useful feature for the customer. This is an example of which of the following?

Which type of estimating is used to improve the accuracy of an activity's duration?