Earlier quoted context omitted.
The study says the participants were 62% female, overwhelmingly ate more vegetables and supplements, exercised more than the control group, were mostly white, as well as self reported glucosamine supplementation - without dosage information. Overall it might be more indicative of wealth, education, and fitness than the effect of glucosamine. The paper itself says it’s not able to differentiate the effect from lifesty…
That is possible but the large sample size may help. The study involved roughly a half million people with around one hundred thousand supplementing with glucosamine. With that many people and these big numbers the results are rather strong.
If the sample is not representative of the broader population, then in no way will any amount of sampling correct that error.
Observational studies often involve quite large samples, but they remain unreliable because it is extremely difficult to address all possible confounding factors. Experimental intervention is the gold standard for a reason, and absent the ethical or practical impossibility of experimentation, should be used here.