Assumptions can be dangerous things. Let’s say you assumed your software was displaying your AVI video file at the proper aspect ratio (ratio of width-to-height). Well, chances are pretty good you’d be wrong…no matter what software you’re using. What!? So the perp in my video evidence may be thinner and taller OR shorter and pudgy-er!? Yep.
Fact – the AVI file format does not provide a standardized way to report aspect ratio. Due to this shortcoming, applications cannot calculate the proper Display Aspect Ratio (DAR) if it is supposed to be different than the Storage/Sample Aspect Ratio (SAR). Therefore, applications "assume" a square Pixel Aspect Ratio (PAR) and display the video at it's SAR, even if that's not correct...which is generally the case when dealing with DCCTV evidence.
Moral of this story? Don’t assume.