History is always someone's opinion

As project managers we always have one eye on the rear view mirror:

• Project history databases provide the benchmarking for new estimates
• Benchmarks set the velocity goals for Agile teams
• Actual data form the coefficients for equations -- usually linear -- that purport to forecast the future (See: earned value linear equations for efficiency and cost/schedule to/at complete)
• Track records are used to vet vendors, contract employees, and justify promotions.

To wit: history is all around us!

And, we note:

History is always someone's opinion!

Thus: history may not be objective. We hear this missive repeatedly: you are welcome to your opinion but not your own facts. Well, swell, if the facts are indisputable, but often they are not. Facts and opinions mix right at the outset, so not only are you welcome to your opinion, but you may be authoring your own facts, welcome or not.

And then comes biases: there are so many boutique biases that it takes pages to define them. So, again, as thinkers, we're not objective, and go for that! It would be unimaginable not to have imagination... of facts and history. In other quarters, it's called revisionism, one of hundreds of "isms" to go along with dozens of biases.

Thus, beware the admonition to look to history for objective facts. We are told the best decision making is fact-based... but only if you are welcome to your own facts!