The Regressive fallacy

This fallacy is committed when the natural fluctuations around an average position are not taken into account when providing explanations of events.

Regression or "regression to the mean (or average)" refers to the tendency of a variable characteristic to move away from extreme values towards the average value.

The regressive fallacy is most commonly committed in relation to chronic illnesses. People will seek out treatments when they are feeling at their worst. Due to "regression to the mean", their condition will improve with time; however, they will attribute their improvement to the remedy.

Examples:

• Skeptics, when stating that there's no proof that, for example, homeopathy works, will often be faced with the answer, "well it works for me".
  
  
• People will swear by their favourite remedy as it improves their condition every time there's a flare up.

In both cases the regressive fallacy can explain what's going on. As can be seen in the diagram below, the blue line represents a typical course for a "flare up" of a condition. The condition is at an average level, then tends to dip to a low as the symptoms get worse before returning back to the average.

When the symptoms are at or around their worst, people will be more likely to take some form of medication. As can be seen from the diagram, any form of treatment or remedy taken at points A, B, or C will be followed by the improvement as the condition returns back to its average. This can lead to the illusion of ineffective treatments actually working.

Note: a remedy taken at point A will see a slight worsening before the improvement. Alternative practitioners put this down to a "healing crisis" - they often claim that the slight worsening of symptoms is due to the body expelling "toxins" and claim that it's proof that their remedy has started working.

Other examples.

1. In sport, football strikers, for example, often go through a hot patch where they score prolifically, followed by a lean spell where they get scrutinised and criticised for being out of form. In reality, the hot and cold spells are just fluctuations around their average.
   
   
2. A golfer may play a few dismal rounds, then he buys some new golf shoes and his scores start to improve. He may well attribute his improving form to the new shoes whilst not realising that his form has just returned to what it was before: his average.
   
   
3. After a particularly high number of accidents on a road one year, the police introduce speed cameras to reduce speeding and, by assumption, accidents. It is found that the number of accidents reduces in the following year. Does this prove that speed cameras are an effective road-safety measure or has the accident rate fallen back to the average?

These are examples of the post-hoc reasoning, and false conclusions, that the regressive fallacy leads to.