Observation Bias

Observation Bias

Paul Arden | Thursday, 7 December 2017

A character who I forgot to mention in last week’s FP (Recognise Anyone?) was the one who insists that it’s impossible to hook a fish on fly gear beyond 75 feet, thus adding to the chorus of ‘distance casting’ being a waste of time and effort. This opinion is undoubtable based on their own experiences; they’ve probably missed a couple of takes when their wind-assisted cast made it out further than they normally manage and, as such, they’ve decided that getting a positive hook-up at distance is highly unlikely. Telling them that I regularly catch fish at distances beyond their ‘limit of plausibility’ falls on deaf ears (I caught a schoolie bass in Jersey this year at well over 100 feet), as they’d much rather hear from similarly minded individuals who also like to bash the ability to cast well. This is a perfect example of observational bias.

Observational bias, for those who haven’t come across the term, is a trait where someone allocates much more importance to information that backs up their strongly held point of view than they do to information that is contrary to it.  The troubling issue is that we’re all guilty of it.  In a recent experiment a group of people had statements that contradicted their beliefs read to them whilst they were in a MRI scanner – the researchers found that this elicited responses from the subject’s brains that were remarkably similar to them being physically attacked, something worth remembering when you get into an argument!

This observation bias pervades all aspects of life so it’s to be expected that, when we talk about fishing and casting, it is also detectable.  I’ve recently started tying the flies that Tracy and I will be using on our next flats fishing trip, so far every fly I’ve tied contains rabbit fur either as a tail or a ‘wing’ on a Charlie or Gotcha type pattern or as a glued down zonker strip on a shrimpy type thing that I tie.  Now I like rabbit fur on my bonefish flies, I think it gives the fly some movement even when fished almost static in front of spooky fish.  On arriving at a flat I’ll almost certainly select something with rabbit fur, in fact it’s now getting hard not to as my fly box is full of them.  However, if I take a step back and look at how I’ve arrived at this situation it’s clear that the answer is down to my own observational bias.  At some point in the past, probably when learning how to bonefish, I’ve had some success with a fly made from rabbit.  This has then has prompted me to use similar patterns on subsequent days, thus every bonefish caught reinforces my view that rabbit fur is great but, perhaps more importantly, failures are shrugged off almost as non-events and are quickly forgotten.

In a similar vein I’m of the opinion that the ‘bonefish special’ fly is next to useless.  It looks like no food item I’ve seen on the flats and it fishes the wrong way up for bottom feeding fish.  I’m sure I’ve fished it a few times over the years and have blanked without fail using it.  In this case the failure is the memorable part as it bolsters my observation bias that this fly is rubbish, irrespective of others telling me that they catch on it.  Actually on our last trip I set myself a goal of catching a bonefish on a bonefish special.  I thought my best chance would be to find a large shoal of small fish and pitch it in to the middle of them (if there’s one thing the fly has going for it is that it’s light and won’t upset a shoal by landing amongst them).  Knowing where to find such a shoal I tackled up appropriately and started to walk to where I thought they’d be hanging.  On the way I spotted two nice sized bonefish heading towards me and thus had no option than to present the useless fly to them with the obvious conclusion – yep, the lead fish nailed it like it hadn’t eaten for a week!

Obviously most experienced bonefishers will tell you that it’s the presentation that counts and not the fly, and I’d totally agree with them, but the observational bias is so deeply set that no doubt the first fly on to my leader next March will be a rabbit fur Gotcha or similar.

As a final note for now on observational bias; working as I do as an empirical physicist I spend a lot of time thinking about experimental configurations and how not let bias creep in.  This often brings me into conflict with the theoretical physicists who want to ‘validate’ their models.  When the experimental results don’t fit with the modellers predictions they’re often forced to specify additional parameters that they haven’t accounted for previously – or ‘fudge the shit out of it’ as the empiricists would call the process.  It can often get to the point where the model only replicates the experimental results over such a narrow parameter space that it’s clear to everyone that the hypothesis is broken (we have another saying: “if you want a straight line result then only make two measurements”).  So what’s the relevance of this?  Casting physics, to me this is still riddled with observational bias, including some often quoted published papers.

Here’s a simple thought experiment for you: suppose someone takes a coin from their pocket and flips it 6 times and each time it lands on heads.  For the seventh flip you are asked to bet on the outcome – what would you pick and why?

Have a great weekend whatever you’re up to.  I’m entered into my annual foray into competition fly fishing – I’ve been promised that because it’s a Christmas match that I’ll be plied with booze all through it so hopefully I’ll forget about the cold.  Other than that I’ll be tying more rabbit fur flies.



Bonefish underwater

James bone LI 2017



sexyloops 3

Tracy bone LI 2017