Diseases notes - Correlations and causal relationships

Correlations and causal relationships

Causal relationships - a logical relationship between one event (cause) and another (effect). There will be a correlation between the cause and effect. 

Correlation - a linear relationship between two variables, where an increase or decrease in one variable coincides with an increase or decrease in the other ariable. However the correlation does not imply causation. There may be another variable which causes both of these variables to change. 

For example numerous studies showed that women who were taking combined hormone replacement therapt (HRT) also had a lower than average incidence of CHD. This lead doctors to believe that HRT was protective against CHD. However controlled trials showed that HRT actually caused a small significant increase in risk of CHD. Re-analysis of the data showed that those undertaking HRT were more likely to be from the socio-economic groups with better than average diet and exercise regimes. The two were…