Let's say that you want to calculate the prior for a woman in her 40s getting breast cancer. P(H)
But instead of using the base rate from women in their 40s getting breast cancer, you use the base rate of men getting breast cancer in their 40s. This will make P(H) extremely low and distort your posterior probability.
What kind of philosophical fallacy has been committed? Is there a name for this one? Reference class fallacy?