If you go to the "methodology" button on that site you will find, among a lot of other info:
Every job in the Salary Wizard has been thoroughly researched and validated by Salary.com's team of compensation consultants who have combined experience of over 40 years in the compensation and statistical analysis fields. Salary.com purchases the most current salary surveys available from well-recognized, reputable compensation consulting firms. This survey data is used for analysis and benchmarking by extracting and reporting the market salary data for each position that matches to one of the jobs reported in the Salary Wizard. To ensure jobs are appropriately matched, our analysts benchmark the jobs based on job content, not job title. Note that all data used in researching salary levels for the Salary Wizard has been reported by human resource professionals. In the Salary Wizard database and in the premium Personal Salary Report and CompAnalyst databases, Salary.com does not use any salary information from individual site users, placement agencies, job postings, nor any other sources that would traditionally be characterized as "unreliable" by compensation or human resource professionals.
The Salary.com research database includes information on more than 1.3 million incumbents and 5,000 companies. Most jobs in the Salary Wizard are based on 100 or more incumbent salaries.
The salary data is presented in two pieces: base pay and total cash compensation (base pay plus annual incentives). The market pay level is based on the median, or 50th percentile, of all salaries reported for a given job. This represents the midpoint of the competitive market rate for that job. To provide perspective on scope and distribution of pay amounts, the Salary Wizard also shows the range around the median that includes the half of the people in that job who are paid closest to the market median - and thus excludes the lowest 25 percent and the highest 25 percent by pay.
Results for each piece are displayed in a graph to show visually this "interquartile range." The minimum of the range is the 25th percentile, which means only 25 percent of salaries reported for a particular job fall below this level; the maximum of the range is the 75th percentile, which means 75 percent of all salaries reported for that job fall below this amount (i.e., 25 percent fall above this amount). Although this interquartile range roughly equates to the typical market range for the position, there are a reasonable number of people whose pay is higher or lower than the interquartile range - in fact, 25 percent of those in any given job are expected to be paid above the 75th percentile and 25 percent are expected to be paid below the 25th percentile.
Although the data sources we use are the most recent available, there is a lag between the effective data of the salary information they report and today - sometimes more than a year. To recognize that salaries increase faster than studies are printed, Salary.com uses the industry standard approach of modifying the data by applying an aging factor to adjust the data to a common date and to accommodate the movement of salaries over time. Not all salaries move at the same rate. For instance, in the last few years, salaries in the information technology field have increased much faster than salaries in other jobs (5 to 15 percent for IT versus 2 to 5 percent in general). Therefore, IT salaries are adjusted at a higher rate than non-IT jobs.