The prompt neutron decay constant in a steady-state subcritical system can be directly measured using a reactor noise analysis method such as the Feynman- method. To reduce the nuclear data–induced uncertainty of for a target system, this study investigates the applicability of data assimilation techniques, i.e., the bias factor method and the cross-section adjustment method, based on a subcritical measurement of conducted at Kyoto University Critical Assembly (KUCA). The sensitivity coefficients of and with respect to the nuclear data were efficiently estimated using a deterministic SN transport code with first-order perturbation theory. As a result, the a priori relative uncertainty of due to the 56-group SCALE covariance data can be reduced if there is strong correlation between the measured and the target . The experimental value of contributes to improving the nuclear data of total fission spectrum and total fission neutron number via strong correlations between and prompt and between and prompt , by utilizing the sensitivity coefficients of with respect to prompt and .