In order to control for experimenter as well as confounding variables such as severity of condition, attitudes, age, etc., researchers assign subjects to groups by random assignment.

The answer to this question is the random assignment to control experimeter BIAS. This is the method or the only way in order to control for experimeter BIAS as well as confounding variables such as the severity condition, the attitudes, the age and etc., The researchers or the person who conduct subjects to groups by random assignment.

The horse latitudes-high-pressure zones about 30 degrees north and 30 degrees south of the equator.-warm air traveling away from the equator cools and sinks-weather tends to be clear and drythe doldrums-low-pressure near equator-air rises and spreads out toward the poles-rising moist air produces clouds and heavy rainthe westerlies-blow from the west-move from the horse latitudes toward the poles.-bring storms across much of the united statesthe tradewinds-blow from the east-move from the horse latitudes toward the equator  -strong, steady winds