Chapter 3: Mobile Personal Content Uncovered 57 provide an understanding of personal content features that affect us all, but also to bring up some fundamental topics that need to be addressed when personal content management systems are being developed. Perhaps the most important feature of personal content is its incred- ible growth rate. The amount of digital photos, for example, is growing at an enormous rate: according to one study, it is estimated that from 2004 to 2009, the number of digital photos taken increases by 24% each year (IDC 2006) (Figure 3-4). And this estimate includes only digital photos, not the multitude of other content types. There are two obvious reasons for the explosive growth: 1. New content is continuously created. 2. Existing analogue content is converted into digital form. A typical example of the fi rst category is taking a photo with a digital camera. Good examples in the second category are scanning old printed photos or digitizing vinyl LPs. Two more reasons, not as obvious, can be identifi ed: 3. New content types are born. 4. Existing digital content is transformed into another format. An example of the third category is a playlist (a fi le that contains links to the songs that together form the playlist), which was not a widely used content type a decade ago. Taking a screenshot of a computer Figure 3-4. Estimated digital photo growth rate from 2004 to 2009 (IDC 2006). The values on the y-axis are only indicative, since the estimated number of photos taken in 2004 varies wildly between 20 and 235 billion, depending on the source.