The phrase "substitution method" refers to a problem-solving technique where you replace one part of the problem with another, simpler part, in order to find the solution more easily. Full definition
Water substitution methods, whether using treated wastewater, seawater or gray water, are generally viewed as less controversial than the use of regenerated water, a process in which wastewater is treated and added back to the water supply. (scientificamerican.com)
The person mean substitution method was used in the current study: the individual's mean for the relevant scale / subscale was used to replace the missing values. (link.springer.com)