This model was called a two-step flow, because in step one, a message was sent out through the mass media; in step two, community level opinion leaders translated the message to the public. The model responded to the [MS not equal to MR] anomaly when Katz and Lazarsfeld, political scientists, noticed that political advertising was not making an immediate impact on voter behavior. They theorized, therefore, that voters were influenced not by the mass media, but by the opinion leaders who helped the public understand how to interpret mass media.
The "opinion leader" was the major contribution of the Katz & Lazarsfeld
model.