Telescope Reviews: Period Luminosity relationship and Cepheids

I am writing a term paper on Cepheid variable stars, specifically, on Henrietta Leavitt and her discovery of the period-luminosity relationship of Cepheids, used as a yardstick to measure star distances.
What I cannot seem to find is information on how the intrinsic luminosity of a Cepheid was discovered, so that it could be used together with the apparent luminosity, in a distance modulus formula, to determine the distance in light years.
I guess I am looking for how the math is done. I have done some reading and still cannot find specific information on how the instrinsic luminosity of a Cepheid was determined, either by parallax or by triangulation. All I find is a reference to Harlow Shapley calibrating the P-L relationship with a "statistical parallax method".
Any help, or pointing to sources much appreciated.
Phil

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Zhumell 10" Dob (modified with degree circle and Wixey inclinometer)
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