Just because using an intervention is associated with an improved or worse outcome for someone, that doesn’t mean that the intervention made them better or worse.
Sometimes researchers find an association between something people do – like going to the doctor – and something that happens to them – like being sick. This does not mean that what they did caused what happened to them. For example, it is more likely that people went to the doctor because they were sick than that going to the doctor caused them to be sick.
When there is a link between people using an intervention and improving or getting worse, the intervention may or may not be the cause. The link may have happened by chance or people might have worsened or improved because of something else.
For example, ice cream sales and crime are linked. When more ice-cream is sold, more crime is committed. Does this mean that selling ice cream causes people to commit crime. Of course not. A more likely explanation is that more ice cream is sold when it is hot and more people (criminals and potential victims) are outside, thus more likely for a crime to occur. So limiting or reducing sales of ice cream on hot days (an intervention) is very unlikely to reduce the number of crimes!
BEWARE of claims that an intervention has an effect because using the treatment is associated with people getting better or worse.
REMEMBER: Think about whether you can be sure that there aren’t other reasons for the association.