Correlation or Cause and Effect?

I've stumbled across multiple references to the relationship between the number of hours of Professional Development and teaching effectiveness recently. The other day I saw a reference that claimed research shows that teachers need at least 80 hours of professional development in order to be effective teachers. No time frame was given to achieve this 80 hours. Then I ran across a reference to this "research" again in a web site on teaching in science and math.

At my school we, the teachers, fill out a form at the end of the year listing the workshops and such we attended during the year. The form changed this year. The old form asked for professional affiliations, workshops/meetings attended, travel, college course work, and included an section to list other activities that enriched the teacher that were not covered in the other areas. Our new form asks only for PD Activities and then number of hours in each activity.

Now, I'm not saying that tracking hours of PD is a bad thing. However, this sort of tracking leads to forced PD. My question is this, "Does Professional Development for the sake of Professional Development change education?" Or, is the relationship we see not a cause/effect relationship but a simple correlation. Do those teachers who are intrinsically motivated to constantly improve their teaching simply seek out opportunities for professional growth on their own and easily make it to 80 hours?

I get really tired of correlation data being foisted off as cause effect relationships. I'll have to read more and see if I can find the original research. Any one have any good sources?