The results of these stations are communicated as soon as possible to the research institute. The cost of a survey is lower if several of them are conducted at the same time. Surveys realised for the press are often sold cheaper than those which are not supposed to be released. Lastly, the price of the surveys may be reduced if the client subscribe to a series of surveys. A barometer is a survey the questions of which are repeated, in the same terms, at specified intervals.
Political barometers are often monthly whereas economic barometers can be quarterly.
The great advantage of barometers is that they make it possible to analyse the opinion changes. No doubt, there is no other country in the world where so many surveys are released. About 500 surveys are issued per year. Political polls are especially numerous, even compared to the United States, cradle of surveys. Such wealth of surveys has three causes.