title_body: Does coffee really reduce the risk of Alzheimer's?
upvoted_answer: tl;dr: Not sure yet, but it looks promising
  
  So there are a whole bunch of studies that have been done that have investigated a potential link between coffee consumption and Alzheimers.
  The problem, as always, is that these types of 'cohort studies' are really difficult. Basically the way it works is:
  
  Get a load of volunteers
  By doing questionnaires, work out everything about them
  Track them over a long period of time
  See if there's a statistically significant link between combinations of behaviours (such as drinking coffee) and a particular outcome (diagnosis of Alzheimers)
  
  All the standard stuff applies here: you are not testing for causation, only for correlation. For example, ~80% of Americans drink coffee, but in other cultures this number varies a lot. If the coffee drinkers in your study are predominantly American, and the non-coffee drinkers are predominantly not, you have a whole bunch of confounding factors. Designing your study to avoid this kind of situation is really hard, especially when you're relying on volunteers.
  There's also the issue of what exactly are you measuring? Someone might drink 3 cups a day now, but only has done for the past few years. Another person might drink 2 cups a day, but has done their whole life. Another person might drink 2 cups of decaff a day. Questionnaires are usually not sophisticated enough to capture this level of detail (partly because in your big expensive study you want to capture as much information about different behaviours as possible - not just coffee consumption).
  Remember also that medical studies are chronically badly reported in the mainstream press. Even in 'high-brow' publications, by the time the headline-writers get their hands on a study, the message can have changed wildly. It's always worth trying to dig up the actual paper whenever new research is mentioned in the news. If no paper is linked, just ignore the headline.
  One tool against all of this uncertainty is meta-analysis. This is where you look at every single study, weighted by how confident you are in the data (e.g. by weighting more to studies with more participants, and better questionnaires etc.). You then do some careful statistics work to get a measure of confidence across all of the studies. Think of it as a sophisticated way of 'averaging them out'.
  One of these was done in 2002. Only 4 studies had been done by then, but the conclusion was:
  
  Although our pooled estimates show that coffee consumption is inversely associated with the risk of AD, the four studies had heterogeneous methodologies and results. Further prospective studies evaluating the association between coffee consumption and AD are strongly needed.
  
  (Heterogeneous is science for 'different')
  i.e., &quot;Not sure yet, not enough data, but it looks promising, give us more money for research.&quot;