It also said that it was a retrospective study, where researchers trawl through old data and seek to find statistical trends, rather than examining new patients.

This retrospective study presents a strong correlation between co-payment levels and medication use for these chronic illnesses.

One retrospective study stated that over 80% of patients could state the exact date their headache began.

In a retrospective study of 345 patients with middle ear cholesteatoma operated on by the same surgeon, the overall 5-year recurrence rate was 11.8%.

One retrospective study found that about a third of people made a full recovery, about a third showed improvement but not a full recovery, and a third remained ill.