Scaling the data. GT data are scaled in 2 ways: relative and fixed [12]. The difference between them is the time frame used to normalize the search volume. In relative mode, data are scaled using the average search volume over the time period selected. For example, the search volume index graph for “flu” is normalized using the extrapolated search volume for “flu” from January 2004 to the present. If the time frame is restricted—for example, restricted to 2008—then the data are scaled using the average search volume for “flu” in 2008 as the denominator.
In fixed mode, the data are normalized using the extrapolated search volume at a fixed time point (January 2004), which is when GT data start. Because the denominator does not change, users can look at different time periods and can relate them to each other. A Web search query for the terms being investigated must have been done at the reference time point (January 2004); otherwise, no denominator exists for comparison. To view data scaled in fixed mode, the data must be downloaded in a comma-separated values (.csv) file and subsequently imported into a database or spreadsheet program. This feature is available only to users who are logged into their Google accounts. The news reference volume graph shows the raw number of Web search queries that appeared in Google News stories.