According to Sun Young Lee and Daniel Riffe, the agenda-building theory speculates that the media does not operate within a vacuum. The media agenda in fact is the result of the influences that certain powerful groups exert as a subtle form of social control. Journalists have limited time and limited resources that can contribute to external sources getting involved in the news media's gatekeeping process, and some scholars have attempted to reveal certain relationships between information sources and the agenda the news media has made up, probing who builds the media agenda. There are multiple sources that can participate in this agenda-building process through various different ways, but researchers have been the most interested in the effectiveness of information aids such as media kits and press releases within the news media agenda, and this is a measure of the success of organizations public relations efforts.