$2 \mathrm{NO}+2 \mathrm{H}_{2} \rightarrow \mathrm{N}_{2}+2 \mathrm{H}_{2} \mathrm{O}$

The above reaction has been studied at $800^{\circ} \mathrm{C}$. The related data are given in the table below

Reaction serial number	Initial Pressure of ${H_2}/kPa$	Initial Pressure of $NO/kPa$	Initial rate $\left( {{{ - dp} \over {dt}}} \right)/(kPa/s)$
1	65.6	40.0	0.135
2	65.6	20.1	0.033
3	38.6	65.6	0.214
4	19.2	65.6	0.106

The order of the reaction with respect to NO is ___________.