Dr David McGrath

Spine Physician

MB BS (Hons) FAFOM, RACP, FAFMM
Master of Pain Medicine

                                       Cause and Effect (Part 3) Critical Threshold

To recapitulate.
1. A cause can be regarded as an essential input. No clouds,no rain. By themselves, they are not necessarily sufficient. Clouds need further inputs to create rain.
              We can express that mathematically as A+B..=D (A alone =0)
2. There may be more than one type of effect, having a different causal set. We can have a storm, light drizzle, snow, and hailstones.
            This can be summarised as A+B+...=D1
                                                    A+C+..= D2
(In this case, both D1,D2 share a common input A )

The situation becomes yet more complicated, when we introduce the idea of negative factors, or factors that reduce the possibility of a phenomena.

If a protective factor(E) exists, the phenomena does not occur. A+B-E..=0
This makes prediction extremely difficult. Clouds may be building in the sky, but a protective factor stops any rain. Should the protection disappear, it would act like a trigger releasing the phenomena. Hence we see thunderstorms with lightning. The brake is released.

In general there may be more than one negative input, as there are multiple positive inputs.
                            A+B-E-F....=D or 0
In other words, thresholds exist for all phenomena, depending on the summation of positive and negative inputs. Or expressing it another way, we have
1. Causitive Inputs A+B+..
2. Protective Inputs -E-F-...

The negatives may operate like positive inputs. That is, they are necessary, but insufficient by themselves to create a negative effect.

All of this adds up to complexity, and difficulty sorting out the wood from the trees.
Heart Risk as an Example.
There are a number of possible risk factors for heart disease, none of which may be sufficient.
High BMI=A
Lack of cardiac fitness=B
Infection=C
Lack of vitamin K=D
High crabohydrate diet=M
Cholesterol is a protective factor=-E
              A+B+C+D+M-E=0 (No cardiac event)
As A,B,C,D,M grow so does E. If a cardiac event occurs, A+B+C+D+M-E=D (Heart attack)
A statistician, may well reach the erroneous conclusion that cholesterol was a cause. The confusion is removed with the observation that low cholesterol leads to early death and tends to rise with a struggling system. (U shaped curve ) A true positive input for disease, is a positive correlation only.