The present work reports the first measurement, via glancing incidence X-ray reflectivity (GIXR) and reciprocal space mapping, of the statistics (mean and standard deviation) of the epitaxial layer widths comprising quantum well infrared photodetectors. Both an optimization procedure and GIXR calculations for various hypothetical structures are used to determine that multiple quantum well structure which most likely yields the measured GIXR. We find that the absolute size of the layer width variation is different for different growths, but that the fractional layer width variation is about 2% for three samples studied. An important observation is that this layer width variation is consistent with an effusion cell temperature variation of 1 degreesC during growth. A second result of this work was the derivation of an analytical formula for finding from a GIXR measurement the standard deviation of the epilayer widths about their mean value. Important in this derivation is the fact that at X-ray wavelengths the semiconductor refractive index is less than unity, and is a significant effect on the analysis for small (glancing) incident angles.