system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did the World Health Organization come to determine that eating processed and red meat causes cancer.

I've read some articles, but how exactly did the study gather and use the data to find the correlation between eating processed and red meats and colorectal, pancreas, stomach and prostate cancer?  In statistics, I've always been taught that correlation never shows cause.  Furthermore, what do they mean when they classified red meat as "probably carcinogenic to humans"?
gpt
>correlation never shows cause

I think "proves" is a better word than "shows". Correlation won't prove anything, but it can show a link. Red meat was already thought of as something that could potentially increase the risk for cancer, so the correlation was more an additional argument rather than the entire basis. 

This has long been under investigation because of the potential risk of nitrites, a class of salts commonly used to cure red meats, as being carcinogens. Many decades ago, nitrites were used in much bigger quantities to help cure meat. As technology got better, meat industries were able to use only a fraction of the curing agents to reach a similar level of protection. Going inline with the whole correlation idea, the incident rate of the associated cancers also started to go down.

So, while not necessarily proven, this definitely gives a strong warning flag for red meat as it's *very* closely tied with the statistics. However, even if red meat is actually the cause, the incident rate is quoted as being between 5-18% higher. Since the original incident rate is already fairly low, this slightly higher chance is not necessarily significant enough to warrant never eating red meat again (conversely as where the incident rate of cancer with smokers is a good sign you never want to smoke).