Research Design :  Qualitative , Quantitative and Mixed Methods Approaches / 
John W. Creswell 
 - 2nd ed. 
 - London :  Sage,  2003. 
 - xxvi.246 p,; 
<br/><br/><label>ISBN: </label>0761924426 
<br/><br/><label>LC Class. No.: </label>H62  / .C73 2003