Such common, often intractable nervous disorders as stuttering, nail biting, hair pulling, and muscular tics can be quickly eliminated (if not quite ""in a day""), psychologists Azrin and Nunn contend. Their method--tested on 300 persons of assorted ages--is to introduce a ""competing reaction,"" i.e. a reaction incompatible with the habit. (For nail biting and hair pulling, the competing reaction is keeping the hands away from the face.) This response should be practiced at home in front of a mirror until it ""can be maintained for several minutes without seeming unusual to someone who is watching."" More stringently: ""You also rehearse your new manner of dealing with habits in many different situations by acting out the procedures while imaginging yourself in those situations."" Progress is to be measured by keeping a daily record of the frequency of the habit. The authors claim that, among their patients, ""the average reduction of the habit was about 99.5% at the end of six months."" But such near-unanimity would augur a miraculous breakthrough that, for one, Gerald Jonas' balanced appraisal of Stuttering (below) militates strongly against. Keep your hands away from your face and cross your fingers.