Are political polls reliable?

The degree of reliability that we must give a political survey depends on several factors, such as company, Institute or consultant to conduct the survey, the way as has done it, the use of the results and the source from which we obtain information about their results.
The trap or error can be in several of these aspects, can be a phantom company or "ad hoc" (recognized companies even have an alternative name to sign when do things well to reduce risk), may be carried out with little professionalism is bad, intentionally, or be failure at any stage of the research process.
Additionally you may own responsible for Institute, the client that has hired it or a journalist or Communicator, knowingly or not, provide partial information or directly to distort the results.