Take the case of artificial sweeteners. Randomized studies — where people are randomly assigned to one treatment or another to ensure that no other factors interfere — are considered the gold standard. But randomized trials of sweeteners are often small and brief, which makes it hard to reach reliable conclusions about their long-term effects. The way sweeteners are studied in trials is also often very different from the way people use them in the real world. If people who consume more sweeteners are more likely to have Type 2 diabetes, did the sweeteners cause the diabetes?

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