A true test of the device would have one person hide the sample in one of several identical boxes. The box containing the sample would be chosen randomly by flipping a coin, rolling dice, or pulling a number out of a hat. The person would hide the sample and then leave. Only then would the user of the GT200 and the observers enter the test area and try to find the sample. The user would determine which, if any, box held the sample and report their answer to the observer. The box would not be opened then to check if he was right. The procedure would repeated at least 20 times if four identical boxes are used.
Before testing, the user should be able to test the device in the area, to make sure there is no interference which could be used later to claim the test did not work. Also, the user should be able to test the device by seeing if the GT200 can find the sample explosive when in plain sight.
NO PERSON should watch both the hiding and the searching for the explosive sample!! If they want to set up a camera on a tripod to record everything to ensure fairness and watch the video later, that is a perfect solution. But a person can give away clues as to where the real sample is hidden.
Please Thailand! Use a double blind test!!
An excellent example of such a test, that was used on the same device when it had the name MOLE is listed here: