The variables may supply information on personal characteristics as sociodemographic factors (age, gender, race, marital status,..etc), life style (food consumption, physical exercise, …etc) and health status ( presence of exposure, disease, …etc ). Also, a variable may show geographical location (region, country, city, area, …etc.) or temporal distribution (day, month, season, year, ..etc) . Thus, the first step for any research is to select the list of variables relevant to the study objectives.