A study determined the link between sodium intake and blood pressure in children and adolescents between ages of 8 and 18. The mean systolic blood pressure (SBP) was 112.8 mm Hg among the 636 study participants who were overweight or obese and consumed high levels of sodium, resulting in a 95% confidence interval for this group of 110.7 to 114.9 mm Hg. The numerical value of the margin of error for this confidence interval is __________ mm Hg.