This is an investigation of whether the real world behaves as predicted by the hypothesis. Scientists (and other people) test hypotheses by conducting experiments . The purpose of an experiment is to determine whether observations of the real world agree with or conflict with the predictions derived from a hypothesis. If they agree, confidence in the hypothesis increases; otherwise, it decreases. Agreement does not assure that the hypothesis is true; future experiments may reveal problems. Karl Popper advised scientists to try to falsify hypotheses, ., to search for and test those experiments that seem most doubtful. Large numbers of successful confirmations are not convincing if they arise from experiments that avoid risk.  Experiments should be designed to minimize possible errors, especially through the use of appropriate scientific controls . For example, tests of medical treatments are commonly run as double-blind tests . Test personnel, who might unwittingly reveal to test subjects which samples are the desired test drugs and which are placebos , are kept ignorant of which are which. Such hints can bias the responses of the test subjects. Furthermore, failure of an experiment does not necessarily mean the hypothesis is false. Experiments always depend on several hypotheses, ., that the test equipment is working properly, and a failure may be a failure of one of the auxiliary hypotheses. (See the Duhem–Quine thesis .) Experiments can be conducted in a college lab, on a kitchen table, at CERN's Large Hadron Collider , at the bottom of an ocean, on Mars (using one of the working rovers ), and so on. Astronomers do experiments, searching for planets around distant stars. Finally, most individual experiments address highly specific topics for reasons of practicality. As a result, evidence about broader topics is usually accumulated gradually.
ISIL also captured fighter aircraft in Syria. The Syrian Observatory for Human Rights reported in October 2014 that former Iraqi pilots were training ISIL militants to fly captured Syrian jets. Witnesses reported that MiG-21 and MiG-23 jets were flying over al-Jarrah military airport, but the US Central Command said it was not aware of flights by ISIL-operated aircraft in Syria or elsewhere.  On 21 October, the Syrian Air Force claimed that it had shot down two of these aircraft over al-Jarrah air base while they were landing.