96 Personal Content Experience: Managing Digital Life in the Mobile Age the other users’ interaction with the search results that they have received after making a similar query (Lehikoinen et al. 2006). • Proactive searches based on assumptions of future activity. For example, the system could explore the events in the user’s calendar, try to identify what content is related to the events, and download it to the device or – at least – provide a shortcut to the object. Searching is discussed further in Chapters 5, and from the human perspective in Chapter 6. 4.5.3 Organizing: Sorting, Grouping, and Filtering Metadata allows people to organize their content in a number of dif- ferent ways. Which way is most useful depends on the situation and personal preferences. For instance, although a date is often a good way to organize images, people might prefer to look for content based on events, such as “Holiday in Italy” or “Dad’s birthday”. Users should to be able to change the way content is organized by using different criteria, such as creation time or content type. Organizing includes the following operations: • Sorting is ordering objects based on values of some attribute, known as a sort key. There may be a single sort key (attribute), or more complex sort ordering may be done by combining several keys. The objects can be arranged in ascending or descending order, depend- ing on the values of the sort key and its type (numerical, alphabeti- cal, time). • Filtering is determining whether some objects fulfi l a certain criteria or not. This helps the user to focus on some objects by hiding other objects that do not fulfi l the criteria. Filtering is typically based on checking whether the object’s attribute has a certain value or if the value is within some range. • Grouping is about fi nding some common characteristics (or values of attributes) from a set of objects and arranging these objects together. For example, many current audio players create automatic playlists based on the user’s rating of the songs. The content type affects grouping the objects. Automatic organization is easier for some content types than for others – for instance, personal images and videos versus commercial music. Images are often related to some special occasion (such as birthday or