Causality, Analogy and… Twins?!!!

What a great movie from the eighties. One always thinks of twins as identical or monozygotic. But twins can be dizygotic or fraternal meaning that they develop from two separate eggs and share the same womb. In this case they are more analogous than anything else – as in the movie Twins with Arni and Danny (see the trailer here for a refresher).

In my most recent set of posts I have been talking about Bradford Hill’s criteria for causality – also know as cause and effect (see here for first post). So far we have covered strength, consistency, specificity, temporality, dose-response, plausibility, coherence, and experiment. Today we are going to talk about analogy – the ninth and final criterion.

Again it is an easy one today. Perfect for a Friday. When considering an association for causality one can look for similar relationships and essentially judge by analogy. If causality was shown in similar or analogous evidence to the relationship you’re interested in then this would support your hypothesis. With the effects of acetaminophen (Tylenol) on pain clear to us we would surely be ready to accept similar evidence with another analgesic drug in pain relief.

Bradford Hill’s criteria wrap-up:

None of the nine criteria can bring indisputable evidence for or against your hypothesis of causality and none can be absolutely required. What they can do, with greater or less strength, is to help you decide – is there any other way of explaining the relationship of interest than cause and effect?
That’s it! All nine criteria. Now it’s time to try and apply them to a real life example. Let me know how it goes.
Have a listen to “Ready to Start” by Arcade Fire and…
… I’ll see you in the blogosphere.
Pascal Tyrrell

Prison Experiments and Causality? Whoops!

Guard or inmate? Who would you like to be?

In my most recent set of posts I have been talking about Bradford Hill’s criteria for causality (see here for first post). So far we have covered strength, consistency, specificity, temporality, dose-response, plausibility, and coherence. Today we are going to talk about experiment – the eighth criterion. 

This is an easy one (and it’s a Friday… Perfect!). Can the condition of the association of interest be altered (prevented or ameliorated) by an appropriate experimental / semi-experimental regimen? If so, then this would lend support to the notion of causality. 

That’s it. Now consider the infamous Stanford Prison Experiment that has etched its place in history, as a notorious example of the unexpected effects that can occur when psychological experiments into human nature are performed.The experiment was a study of the psychological effects of becoming a prisoner or prison guard. The experiment was conducted at Stanford University on August 14–20, 1971, by a team of researchers led by psychology professor Philip Zimbardo using college students. Needless to say the experiment got out of hand and participants were harmed in the research process. Whoops! Not good.

The Stanford Prison Experiment led to the implementation of rules to preclude any harmful treatment of participants. Before they’re implemented, human studies must now undergo an extensive review by an research ethics board or institutional review board.


You may be able to show causality by an experimental regimen but at what cost? Be careful and think about research ethics before you leap into an experiment.


Watch the trailer to the movie about the Stanford Prison Experiment to get a better idea of what I am talking about and…

… I’ll see you in the blogosphere!

Pascal Tyrrell

Coherence, Causality… and Space – Time?

Warp speed ahead!

In my most recent set of posts I have been talking about Bradford Hill’s criteria for causality (see here for first post). So far we have covered strengthconsistencyspecificitytemporality, dose-response, and plausibility. Today we are going to talk about Coherence – the seventh criterion. 

The association should be compatible with existing theory and knowledge.  In other words, it is necessary to evaluate claims of causality within the context of the current state of knowledge. What concessions do we have to make in order to accept a particular claim of causality? Too much, too little, or just right?

As with the issue of plausibility, research that disagrees with established theory and knowledge are not automatically false.  They may, in fact, force a reconsideration of accepted beliefs and principles.

In his Special Theory of Relativity, Einstein states two postulates:

1- The speed of light (about 300,000,000 meters per second) is the same for all observers, whether or not they’re moving.

2- Anyone moving at a constant speed should observe the same physical laws.

Putting these two ideas together, Einstein realized that space and time are relative — an object in motion actually experiences time at a slower rate than one at rest. Although this may seem absurd to us, we travel incredibly slow when compared to the speed of light, so we don’t notice the hands on our watches ticking slower when we’re running or traveling on an airplane. Scientists have actually proved this phenomenon by sending atomic clocks up with high-speed rocket ships. They returned to Earth slightly behind the clocks on the ground. 

Not convinced? Watch Einstein’s Time is an Illusion for addtitional insight.

Still not convinced? That’s OK. Often fundamental changes to basic concepts of a scientific discipline take time for people to understand and adopt as a belief. This is referred to a paradigm shift

Bottom line is that the cause-and-effect interpretation of your data should not seriously conflict with the generally known facts of the base of knowledge in your field of study – but there is wiggle room here!

Watch the movie trailer for Coherence to confuse you even more and…

… I’ll see you in the blogosphere.

Pascal Tyrrell

Plausibility, My Dear Watson!

Or was that “Elementary, my dear Watson”? I always get those confused…

Anyway, in my most recent set of posts I have been talking about Bradford Hill’s criteria for causality (see here for first post). So far we have covered strength, consistency, specificity, temporality, and dose-response. Today we are going to talk about plausibility – the sixth criterion. An easy one at that.

For plausibility to exist we need the association of interest to agree with currently accepted understanding of pathological/ biological/ physical processes. In other words, there needs to be some theoretical basis for the association we are considering. While we hope to avoid spurious associations, at the same time, relationships that disagree with current understanding is not necessarily false; they may, in fact, be a needed challenge to accepted beliefs and principles.

 As Sherlock Holmes advised Dr. Watson, ‘when you have eliminated the impossible, whatever remains, however improbable, must be the truth.’

Next time we will talk about the 7th of nine criteria: coherence.

Don’t remember who Sherlock Holmes is? See the trailer to Robert Downey‘s rendition of Sir Arthur Conan Doyle‘s famous detective here and…

… I’ll see you in the blogosphere,

Pascal Tyrrell

A Spoonful of Sugar… Makes the Dose-Response Go Around?

An oldie but a goodie! Haven’t heard of Mary Poppins or her spoonful of sugar? Have a peek here for your dose of the classics. In my most recent set of posts I have been talking about Bradford Hill’s criteria for causality. So far we have covered strength, consistency, specificity, and temporality. Today we are going to talk about biologic gradient or dose-response.

This criterion is pretty easy to understand. An increasing amount of exposure increases a risk in question and with a dose-response relationship present, it is strong evidence for a causal relationship!  

Let’s say you think that being out in the sun in a bathing suit causes your skin to suffer a sunburn. So, the exposure is sunlight and the outcome is sunburn. Based on everyone’s experience, it would certainly appear that the longer you stay out in the sun the greater the risk you will suffer a burn! Your parents have been warning you of this for ever.

However, as with specificity, the absence of a dose-response relationship does not rule out a causal relationship.  A threshold may exist above which a causal relationship is present.  

Next time we will talk about the 6th criterion: plausibility.

Watch one of Simon’s Cat earlier clips Cat-Man-Do and see if you can spot the dose-response…

… and I’ll see you in the blogosphere.

Pascal Tyrrell