For \( z = a^{2} x^{3} y^{\frac{1}{2}} \), where '\( a \)' is a constant. If percentage error in measurement of '\( x \)' and '\( y \)' are \( 4\% \) and \( 12\% \), respectively, then the percentage error for '\( z \)' will be ____________ \( \% \).