Confounding is due to a third (or more) variable(s), which means that the distortion is caused by the actual influence of variables other than the hypothesized exposure of interest or the outcome of interest.

Which tool can be used to operate a natural gas supply valve?

a. Strap wrench b. Flat-head screwdriver c. Hydrant wrench d. Adjustable wrench

During calibration of a Clark polarographic O2 analyzer, you cannot get the sensor to read 100% when exposed to 100% O2, even after adjusting its calibration control. The unit has new batteries

Which of the following actions would be correct? a. Use it only with low FiO2 values. b. Send the device out for repair. c. Replace the unit's batteries. d. Change the sensor or electrode.