Public opinion can be divided into four categories: expert opinion; informed opinion; affected opinion; opinion not falling into any of the categories. The importance of these categories is likely to vary according to the issue, those involved, general awareness of the issue, the extent to which opinion is organised through political parties and pressure groups. Lane and Sears argue that public opinion has four major characteristics: direction, intensity, salience, and consistency. They suggest that experience provides the testing ground for values, leading individuals to evaluate the various elements in the communication process. The mass media undoubtedly play a crucial role in the formation of public opinion. Denis McQuail has produced a comprehensive typology of media effects. The role of the media in agenda-setting also cannot be doubted. More recently, the media has played a major role in moving the complex of issues concerning AIDS up the political agenda, especially the plight of haemophiliacs who had become HIV-positive through blood transfusions.