I've learned about many types of study design including experimental (randomised control trials etc.) and observational study types (cohort studies, cross sectional studies, case-control studies) etc. but a lot of the research articles I'm reading do not seem to fit into any of these design types and am not sure what I'm doing wrong. Basically, the article linked above involves radiologist doctors rating x-ray images based on their quality (sharpness, amount of noise etc.). Some x-ray images have been obtained u sing 30% less radiation and some have been obtained using 20% less radiation than a standard dose image. And they want to see if these lower radiation dose images are of acceptable quality. The x-rays were taken on a phantom "dummy", not real patients. There was no random allocation, so can't be an RCT etc. yet it is experimental. I feel like I'm missing something and that the study designs I've been taught about don't fit studies like this? Any help would be appreciated!