Positron annihilation is a suitable and sensitive tool for studying vacancies in semiconductors. All positron techniques provide the positron trapping rate as an experimental result, which is proportional to the concentration of the vacancy under investigation. The proportionality constant is called positron trapping coefficient. It depends on the charge state of the vacancy, on the sample temperature, and eventually, at very high carrier concentrations on the Debye screening length. The application of an independent method for measuring the defect concentration is required to determine the trapping coefficient experimentally. A review of the available experimental data is presented. Only such experiments, where both the positron measurement and the corresponding reference method were performed at the very same sample are considered. Most of the experimental values for negative vacancies found via various reference techniques in elemental and compound semiconductors were found to be close to 1x10¹⁵ s^-1 at room temperature. The trapping coefficient for neutral vacancies is about a factor of two smaller. No positron trapping has been observed for positively charged vacancies and thus only an estimate of the upper limit of the trapping coefficient, 1x10¹⁴ s^-1, can be given.