SkepticismAt WorkRecognizing
Logical FallaciesSteve Cena
2013
Basic Definitions
Cynicism: To doubt without evidence. To dismiss a claim out of hand.
Skepticism: To doubt based on the evidence or the lack thereof.
Faith: To believe without evidence or in spite of contrary evidence. “Faith is believing what you know ain’t so.”*
Belief: To accept based on the available evidence.
* Mark Twain
“I’m a skeptic not because I do not want
to believe but I want to know.”
Michael Shermer, The Believing Brain Prolog
Discussions of belief & skepticism are often centered on paranormal or
pseudoscientific topics
Business Claims
• We need a fresh set of eyes to work on this problem.
• We tried that before & it didn’t work.• The test is 99% accurate.• It’s just an outlier.• I can’t explain why but whenever we do it this
way it works.
Which Claims Should We Believe?
WE SHOULD ONLY BELIEVE THINGS FOR
WHICH THERE IS ADEQUATE EVIDENCE.
What is adequate evidence?
How do we evaluate the data?
DATA
JudgmentsReasoning
Judgments = OpinionsReasoning = Logic
Errors in Reasoning = Logical Fallacies
Claims are made on what seem to be good arguments but these arguments are based on incorrect logical principles. Thus, the claims are not valid.
Ad Hominem Attack
Ad hominem is Latin for "to the person”
• Not invented here
• Outside expert error
• Guilty by group association
False Dichotomy(The Excluded Middle)
• “You are either for us or against us.”
• "You are either part of the solution or part of the problem.”
Arbitrarily reducing the set of probabilities to only 2.
• "It's either the design or the process."
Response or Problem
Measurement Method Material
Machine Mother Nature Manpower
Response or Problem
This
That
Anecdotes Do Not Equal Evidence• Memory is fallible & malleable
– Memories are reconstructed not replayed
• Anecdotes rely on intuition & subjective interpretation
• “… we are apt to want our version of the truth, rather than the truth itself, to prevail.”*
• Anecdotex ≠ Data– The plural of anecdote is not data.
• Show me the data * Steven Pinker, How the Mind Works, pg. 289
Conjunction Fallacy
• 80% of the time delivery is late• 80% of the time the quantity is wrong• What is more likely?
– The next delivery will be late– The next delivery will be late and the wrong
quantity
The tendency to think 2 conclusions are more likely than either conclusion
alone.
Conjunction Fallacy
• 80% of the time delivery is late– P(late) = .80
• 80% of the time the quantity is wrong– P(quantity) = .80
• P(late) x P(quantity) = .80 x .80 = .64
Base Rate Fallacy
When considering the probability of something happening failing to consider
the probability that it can happen.
Base Rate Fallacy
Test 100 acceptable parts99 are correctly identified
1 incorrectly identified
Test 100 unacceptable parts99 are correctly identified
1 incorrectly identified
Base Rate Fallacy
99% accurate!99% of shipped product is acceptable!
99% of rejected product is unacceptable!
Only if half of the product made is unacceptable.
Base Rate Fallacy
Produce 10,000 units
95% are acceptable
10,000 x .95 = 9,500 acceptable units10,000 – 9,500 = 500 unacceptable
Base Rate Fallacy
9,500 acceptable units tested at 99% accuracy
9,500 x .99 = 9,405 accepted 95 incorrectly rejected
Base Rate Fallacy
500 unacceptable units tested at 99% accuracy
500 x .99 = 495 rejected 5 incorrectly accepted
Base Rate Fallacy
9405 correctly accepted + 5 incorrectly accepted 9410 total accepted units
What Shipped
9405/9410 = .9995Considering the BASE RATE
99.95% of shipped product is acceptableNOT 99% as claimed.
Base Rate Fallacy
495 correctly rejected + 95 incorrectly rejected 590 total rejected units
What is Rejected
495/590 = .839Considering the BASE RATE
83.9% of rejected product is unacceptableNOT 99% as claimed.
Post Hoc Ergo Propter Hoc
• Assumption: if A precedes B then A must cause B
• Coincidences happen• Correlation ≠ Causation
– “New York City Has Most Millionaires In The Country” *
• By What Mechanism?
Latin for "after this, therefore because of this“
* http://www.huffingtonpost.com/2010/08/04/new-york-city-has-most-mi_n_670474.html#s122573&title=New_York_667200
Post Hoc Ergo Propter Hoc
XKCD
Regression to the mean
An extreme measurement will tend to be followed by a measurement that is closer to the mean.
Regression to the mean
Regression to the mean
Regression to the mean
Regression to the mean
Regression to the mean
Regression to the mean
Why do you believe?
• Is it because the belief is comfortable or convenient?
• Does the data reaffirm what you already "know" to be true?
• Are contrary data ignored or rationalized away?
• Did you objectively analyze the data and come to this conclusion?
What would change your mind?
• NOTHING!• Falsifiable claim
– “The Scientific method consists of the use of procedures designed to show not that our predictions and hypotheses are right, but that they might be wrong”*
• Rabbit in the Precambrian**
What is your rabbit?* Mistakes Were Made, Tarvis & Aronson**J.B.S. Haldane
What would you do if you’re wrong?
• Minimize – It’s not that bad• Mobilize – Moving target• Optimize – (POB) Pervasive Optimistic Bias• Rationalize – Justify• Invoke any logical fallacy
What would you do if you’re wrong?
Why do you believe?
What would change your mind?
A Short Story
MinimizationMobilization
Argument From Authority
Rationalization
RationalizationStraw Man Argument
Argument From IgnoranceOptimize (POB)
