Minimum rule?

Ever wonder where the 0.05 probability level number was derived?  Ever wonder if that is the best number?  How many of you were taught in your introduction to statistics course that 0.05 is the probability level necessary for rejecting the null hypothesis of no difference?  This confidence may be spurious.  As Paul Bakker indicates in the AEA 365 blog post for March 28, “Before you analyze your data, discuss with your clients and the relevant decision makers the level of confidence they need to make a decision.”  Do they really need to be 95% confident?  Or would 90% confidence be sufficient?  What about 75% or even 55%?

Think about it for a minute?  If you were a brain surgeon, you wouldn’t want anything less than 99.99% confidence;  if you were looking at level of risk for a stock market investment, 55% would probably make you a lot of money.  The academic community  has held to and used the probability level of 0.05 for years (the computation of the p value dating back to 1770).   (Quoting Wikipedia, ” In the 1770s Laplace considered the statistics of almost half a million births. The statistics showed an excess of boys compared to girls. He concluded by calculation of a p-value that the excess was a real, but unexplained, effect.”) Fisher first proposed the 0.05 level in 1025 and established a one in 20 limit for statistical significance when considering a two tailed test.   Sometimes the academic community makes the probability level even more restrictive by using 0.01 or 0.001 to demonstrate that the findings are significant.  Scientific journals expect 95% confidence or a probability level of at least 0.05.

Although I have held to these levels, especially when I publish a manuscript, I have often wondered if this level makes sense.  If I am only curious about a difference, do I need 0.05?  Oor could I use 0.10 or 0.15 or even 0.20?  I have often asked students if they are conducting confirmatory or exploratory research?  I think confirmatory research expects a more stringent probability level.  I think exploratory research requires a less stringent probability level.  The 0.05 seems so arbitrary.

Then there is the grounded theory approach which doesn’t use a probability level.  It generates theory from categories which are generated from concepts which are identified from data, usually qualitative in nature.  It uses language like fit, relevance, workability, and modifiability.  It does not report statistically significant probabilities as it doesn’t use inferential statistics.  Instead, it uses a series of probability statements about the relationships between concepts.

So what do we do?  What do you do?  Let me know.

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