A method of generating program analysis data for analysing the operation of a computer program. The method comprises, executing an instrumented process of the computer program to define a reference execution of the program, intercepting a call to a library function by the instrumented process, executing the library function in an uninstrumented process, for the uninstrumented process, capturing in a log, only data generated by or modified through the execution of the library function required by the instrumented process to continue execution of the program, and wherein the captured log is arranged to enable deterministically reproducing the effect of the library function call on the instrumented process upon re-running of the reference execution based upon the captured log to generate the program analysis data.