1. A method of testing at least one of a process and a source of data for determining a particular condition thereof, comprising the steps of: collecting data from at least a first and second source of data to redundantly detect at least one variable of the process to provide a first signal from said first source of data and a second signal from said second source of data, each of said signals being characteristic of the at least one variable; providing frequency domain transformation data for each of said first and second signals at a given time; obtaining a frequency domain difference function characteristic of the difference pairwise between said frequency domain transformed first signal and said second signal; operating on the frequency domain difference function using computer means for applying a statistical analysis technique to determine a combined statistical frequency spectrum data set for at least one of the process and the source of data; and repeating the steps of providing frequency domain transformation data for a new incremental time position until generation of a three-dimensional surface characteristic of the condition of at least one of the process and the source of data.