I am aware of the Nova classification. "Vague" was the wrong word for me to use. "Overly broad" is probably closer to what I mean. Just read Group 4's definition. If "ultra-processed" can mean a million different things, then it doesn't matter that you've precisely defined all the things it can mean, your study will still be conflating a million independent variables.

If you think emulsifiers are unhealthy, conduct a study on emulsifiers. If you think the absence of Nova group 1 foods is unhealthy, study that. I am questioning the value of studying foods with emulsifiers OR no group 1 ingredients OR added sugar OR were extruded OR were moulded OR which have "sophisticated packaging" OR have fruit juice concentrates OR hydrogenated oils OR etc. etc. etc., as if they formed a single scientifically meaningful group.