Answer:
Instrumentation validity
Step-by-step explanation:
The potential threat that exists in this scenario, is Instrumentation validity. This happens when different instruments are used to test the same thing. There exists the possibility that some of those instruments are not necessarily measuring what they are supposed to be measuring, correctly. This ultimately gives mixed results and can ruin a study. Which is what is happening in this scenario as all the volunteers are using their own "separate" personal equipment.