Tangent: What does it matter how reliable your demographic data is when you're doing digital marketing?
At the end of the day what matters is your ROI from placing ads; whether they are shown to the demographic you had in mind or not is more of a detail (you might think 24-30 women should be your best demographic, but maybe the real decision function is non-linear and more fuzzy algorithms get you a better ROI)
Determining ROI in marketing is a famously hard problem. As the old saying goes, "Half the money I spend on advertising is wasted; the trouble is, I don't know which half".
