posted by Liz .
It is ok to have a negative percent error? I got a negative value for the percent error of the enthalpy of the formation of methane gas
Sure. If the experimental value is less than the actual value the percent error is negative. If the experimental value is larger than the actual value the percent error is positive. But most (at least many) people use the absolute value of the error when reporting in which case + or - makes no difference.