Lecture notes on causality in Evidence Based Health
Imagine an elderly lady walking down an icy path. It snowed the day before and the path is slippery. She is wearing a footwear where the sole is somewhat worn out and she slips on the footpath. She fell and broke her hip. A few passersby helped her to a car and she was sent to the hospital. In the hospital she told her doctor that she had a head injury many years ago and as a result, she always felt wobbly. The question is, what caused her hip fracture?
Was it:
- The ice that was left on the path because the Council did not remove it?
- The head injury she sustained many years ago?
- Her footwear?
The correct answer is, you can refer all the three as possible causes. A cause of an event is defined as an entity, or another event that precedes the event and the event would not have occurred if the cause was not present. Ken Rothman has argued that we have an intuitive notion about cause and effect from the time we are born (Rothman, Kenneth J and Greenland, Sander 2005).
When an infant wants milk or feels hungry, he cries and knows that this act of crying will get him his desired milk or bring him closer to his mother. Similarly, as children grow up, they experiment and they for instance, know, that pressing of a switch brings about glowing of a bulb and therefore associates that it is the switch that causes the bulb to glow. As a result, for many adults, this is a common notion of the cause and effect in life. However, in health care, and in particular in evidence based health care, this notion of cause and effect is problematic as causes can not only precede effects by many years, but as well, many causes are possible for the same health effect and each cause can contribute a little to the overall health effect we get to see. For example, take the case of the elderly woman. The woman felt wobbly, this made her prone to fall; her footwear had little grip, that made it worse; finally, the pavement was icy and slippery. All of these things came together to play their roles in her falling and breaking her hip. Each of these would be a component in some way to cause her falling over. This is presented in the following figure.