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

Just in the Nick of Time… Causality.

Now that was a great movie: Interstellar. See the trailer here for a refresher. So this movie talked a lot about worm holes – essentially an area of warped spacetime. Theoretically a worm hole could allow time travel. Want to know more? Grab a large coffee and see here. You may be thinking what all this has to do with medical imaging but, believe it or not, I posted about x-rays in space earlier in the blog (see here).

Listen to Hans Zimmer’s – Time from the movie Inception (another great movie) to get into the mood.

Now, we have been talking about Bradford Hill’s criteria for causality and today we are addressing the fourth one: temporality. The exposure of your association of interest should always precede in time the outcome.  If factor “A” is believed to cause a disease,  then factor “A” must necessarily always precede the occurrence of the disease. So for example the act of smoking (or being exposed to second-hand smoke) must precede the development of lung cancer for the relationship to be considered causal. This is the only absolutely essential criterion (out of nine).

Easy one, right? Next time I will be talking about biological gradient.

I am not sure you need time to decompress today as it has not been too taxing… but listen to Bonnie Raitt Nick of Time anyway…

… and I’ll see you in the blogosphere.

Pascal Tyrrell

Be More Specific… Or It May Not Be Causal?!!!

Well, if you are relaxed and heading nowhere in particular then I guess you probably won’t be too concerned with showing causality either. In our past few posts we have been discussing Bradford Hill’s criteria for determining causality (see Strength and Consistency for a refresher). If you are stressed out already, have a listen to “Come the morning” from an up and coming Canadian artist from Winnipeg, Manitoba – Sebastian Owl – before reading on.

Today we will talk about the third of the nine Hill criteria: Specificity

When considering the specificity of the association of interest, we wish to establish whether a single putative cause produces a specific effect. When specificity of an association is found, it provides additional support for a causal relationship.  But keep in mind that very often the effect under investigation may have more than one cause. So the absence of specificity in no way negates a causal relationship. This criterium of Hill’s is considered to be the least important and can often be over-ruled in the case of multi-causal relationships.

Next, post we will talk about the oh-so-important criterium: temporality.

If you are nowhere in particular then you are not being specific to your whereabouts – right? Anyway, why don’t you watch this great Film festival short by Mason Cardiff, Nowhere in particular, to decompress and…

… I’ll see you in the blogosphere.

Pascal Tyrrell

Consistent Associations May Be Causal Ones…?

Do you remember the Rain Man movie with Dustin Hoffman and Tom Cruise? Great movie that introduced Savant Syndrome to theater audiences all over the world. The savant syndrome is a rare condition in which persons with autistic disorder or other mental disabilities have extraordinary skills that stand in stark contrast to their overall handicap. There is a very interesting documentary on Kim Peeks who was the inspiration for the movie here. Anyway, last post we talked about strength – one of Bradford Hill’s criteria for causation (see here for a refresher). Today we will talk about consistency, a good qualifier for the often obsessive and ritualistic behaviors of autistic savant persons.

An association between two entities is consistent when results are replicated in multiple studies in different settings using different methods. So if a relationship is causal, we can expect to find it consistently in different studies and among different populations.  This implies that many studies need to be done before meaningful statements can be made about any causal relationship.

A great example of this is the long debated causal relationship between smoking cigarettes and lung cancer. It took hundreds, if not thousands of highly technical studies and many, many publications before a definitive conclusion could be made that cigarette smoking increases the risk of cancer and in a causal manner (see here for a statement from the CDC Surgeon General).  

So be consistent in your smoking cessation and you will consistently avoid the risk of lung cancer…

Next post we will tak about Bradford’s third criterium: specificity.

Relax listening to the very eighties styled theme music to Rain Man and…

… I’ll see you in the blogosphere.

Pascal Tyrrell

Causality and Who’s Running Up That Hill?

Yes, back to the eighties. They were my high school and undergrad years – so very memorable! This song – Running Up That Hill – by Kate Bush was her first great hit from that time.

So, why was she running up that hill you ask? Well, it was because she had finally come to realize the importance of establishing the minimal conditions needed to establish a causal relationship between two entities, of course! Somewhat like the story of Archimedes who leapt from his bath yelling “Eureka” in excitement having discovered a law of physics that would later become the building block to fluid mechanics (see Archimedes principle).

In 1965 (no, I was not born yet – but just!), Austin Bradford Hill a British medical statistician proposed minimal conditions needed to establish a causal relationship between two entities. These later became know as the Hill’s Criteria. Very often people get the relationship of association confused with that of causality. See my previous post Rebel Without a Cause for some insight on when an association can be considered as cause and effect.

Today we will talk about the first of the nine Hill criteria: Strength

– The strength of an association is defined as the size of a given association as measured by appropriate statistical tests.  The stronger the association, the more likely it is that the relation between the two entities of interest is cause and effect.  For example, the more highly correlated hypertension is with smoking, the stronger is the relation between the exposure, smoking, to the outcome, hypertension. Though we cannot be sure of the direction of the relationship (this will be achieved when we discuss Temporality) – as hypertension could hypothetically lead subjects to smoke – we can certainly decide that the strength of the association observed supports our argument of causation.

Look at that, we have completed the first criterium all ready! Next we will look at Consistency.

Have a listen to “Strength Of A Women” by Shaggy to recover from today’s fun and…

… I’ll see you in the blogosphere.

Pascal Tyrrell