# Association VS. Causal relationships

| category RStudy  | tag GWAS

### Association

• When two variables are related, we say that there is association between them.

When researchers find a correlation, which can also be called an association, what they are saying is that they found a relationship between two, or more, variables.

### Causal

• One variable has a direct influence on the other, this is called a causal relationship.
• Causality can only be determined by reasoning about how the data were collected.
• The data values themselves contain no information that can help you to decide.

If two variables are causally related, it is possible to conclude that changes to the explanatory variable, X, will have a direct impact on Y.

If one variable causally affects the other, then adjusting the value of that variable will cause the other to change.

Obviously, it is much more difficult to prove causation than it is to prove an association.

### Non-causal

In non-causal relationships, the relationship that is evident between the two variables is not completely the result of one variable directly affecting the other.

• Two variables can be related to each other without either variable directly affecting the values of the other.

If two variables are not causally related, it is impossible to tell whether changes to one variable, X, will result in changes to the other variable, Y.

### Observational studies

• In observational studies, there is usually the potential for a lurking variable to underlie any observed relationship, so it is difficult to interpret relationships.

Data are collected in an observational study if we passively record (observe) values from each unit.

### Experiments

• In a well designed experiment, there is little chance of lurking variables driving the observed relationships, so any relationship will be causal.

In an experiment, the researcher actively changes some characteristics of the units before the data are collected. The values of some variables are therefore under the control of the experimenter. In other words, the experimenter is able to choose each individual’s values for some variables.