THE FOLLOWING DESCRIBES THE COLUMNS of  xerop.dat:

	ID                                                          
	respiratory infection                                       
	intercept                                                   
	age in months                                               
	xerophthalmia                                               
	cos(time)                                                   
	sin(time)                                                   
	sex                                                         
	height for age                                               
	stunted?                                                    
	time                                                        
	baseline age
	season                                                
	time*time

REFERENCE:  Zeger and Karim, JASA (1991)