A method for analyzing an abnormal particle population in an experimental sample containing particles which involves a particle property distribution index based on an average particle property (e.g., size) distribution curve (10) for multiple normal samples in a particle analysis instrument. The process involves producing an experimental particle property distribution curve by analyzing an experimental sample containing particles in the instrument. The distribution curve of the experimental sample is analyzed to identify particle populations at the lower (14) or upper (18) region of the experimental distribution curve that differ from particle populations in the regions of an average particle property di stribution curve. Thereafter, the number of particles within the lower or upper region of the experimental distribution curve that differs from the average curve is determined and employed for diagnosis of a relevant characteristic of the particle population. A computer program which makes calculations for such methods can be employed or can be integrated into a particle analysis instrument. The method, program, and instruments of this invention provide a novel red blood cell parameter.