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Correlation refers to a statistical relationship between two continuous variables. In common speech if two factors are "correlated" it is often incorrectly taken to mean that one factor causes the other (see [[cause and effect]]). Importantly, however, a correlation only means that two factors tend to occur together and is not proof of causation. The co-occurrence of two factors may arise because one of them causes the other, but a correlation can also occur because both are caused by an unknown third factor (see [[correlation is not causation]]). For example, the prices of two unrelated goods might increase during a period of [[inflation]]; the two prices are correlated with each other because the co-occur but were both caused by the same third factor.

Correlation refers to a statistical relationship between two continuous variables. In common speech if two factors are "correlated" it is often incorrectly taken to mean that one factor causes the other (see [[cause and effect]]). Importantly, however, a correlation only means that two factors tend to occur together and is not proof of causation. The co-occurrence of two factors may arise because one of them causes the other, but a correlation can also occur because both are caused by an unknown third factor (see [[correlation is not causation]]). For example, the prices of two unrelated goods might increase during a period of [[inflation]]; the two prices are correlated with each other because they co-occur but were both caused by the same third factor.

Once a correlation has been noted, it is often possible to determine if there is a causal relationship between the variables, though this usually requires additional research by scientists. Research is particularly necessary if there is not a clear ordering of the factors in time. For example, a correlation between the heights of fathers and sons may be produced by fathers directly causing their sons' heights (e.g. through genetics) but is unlikely to be the result of sons causing their fathers' heights. The latter would require a factor (i.e. height of sons) to cause something that occurred earlier in time (i.e. the height of the fathers), and is therefore implausible. In contrast, a correlation between cancer and consumption of alcohol might imply that alcohol causes cancer but could also imply that cancer causes the consumption of alcohol, for example to reduce the pain of the cancer. As there is no logically necessary ordering to these two factors (i.e. the cancer or the consumption could have started first) additional research is necessary to determine causation.

Once a correlation has been noted, it is often possible to determine if there is a causal relationship between the variables, though this usually requires additional research by scientists. Research is particularly necessary if there is not a clear ordering of the factors in time. For example, a correlation between the heights of fathers and sons may be produced by fathers directly causing their sons' heights (e.g. through genetics) but is unlikely to be the result of sons causing their fathers' heights. The latter would require a factor (i.e. height of sons) to cause something that occurred earlier in time (i.e. the height of the fathers), and is therefore implausible. In contrast, a correlation between cancer and consumption of alcohol might imply that alcohol causes cancer but could also imply that cancer causes the consumption of alcohol, for example to reduce the pain of the cancer. As there is no logically necessary ordering to these two factors (i.e. the cancer or the consumption could have started first) additional research is necessary to determine causation.