The EPA published a study in 2014 on the effectiveness and accuracy of disposable radon test kits in a controlled radon environment. See full report. They found only 40% of alpha track devices, 64% of activated charcoal, 61% of liquid scintillation and 75% of electret ion tests passed. Passing means the results were within +-25% of the real value.
Sun Nuclear Radon Monitors
Sun Nuclear is the leading radon machine used for radon testing. As reported by the company, the accuracy is only +-25% of the actual radon levels.
RadStar Radon Monitors
RadStar radon machines (as reported by manufacturer) have an accuracy of +-10%. Thats over twice the accuracy of the leading radon machine and all radon vial testing methods. This is the only radon machine we use.