Find a Health Sci/Medical Library and get this article:

Mikuriya, Tod H. and Aldrich, Micheal R., "Cannabis 1988 Old Drug, New
Dangers, The Potency Debate", Journal of Psychoactive Drugs, Vol 20(1), Jan-
Mar, 1988 pg 47.

Summary and Conclusions:

Observation of the real world of social and marijuana use,
where autotitration is the norm, renders the scare tactics of
the _new_marijuana_ proponets not only inaccurate but 
irrelevant (*). There is much published evidence about the
availability of highly potent varieties of cannabis from the
ninetheenth century through the present day.  The effects
attributed to the _new_marijuana_ are the same ones debated
for centuries in many different cultures.  The assertion that
"all marijuana research to date has been done on 1 or 2 
percent THC material" (Cohen 1968) ignores several thousand
years of human experience with the drug.  The old 
medical cannabis extracts were stronger than most of the
forms now available, though the potency of illicit hash oils
by the mid-1970's was approaching the level of medicinal 
preparations available before their removal from the USP.
    While it may be true that sinsemilla is more widely
available than 10 or 15 years ago, its potency has not
changed significantly from the 2.4 to 9.5 percent THC
materials available in 1973-1974 (see Table I), or the five
to 14 percent sinsemilla of 1975 (Perry 1977).  The range of
potencies available then (marijuana at 0.1% to 7.8% THC,
averaging 2.0% to 5.0% THC by 1975) was approximately
the same as that reported now.  With such a range, the
evidence simply cannot support the argument by Cohen
(1986) that marijuana is "ten or more times more potent
than the product smoked ten years ago."  And to say that
marijaua potency has increased 1,400 percent since _any_
date in history is patent nonsense.
    It is not legitimate to imply that _average_ low potencies
represent the _full_range_ of potencies available in reality.
Neither is it valid to cite the _low_end_of_the_range_then_ as a
baseline to compare with the _high_end_of_the_range_now_.
The claimed baseline for THC content in the early 1970's
would appear to be too low, probably because confiscated,
stored police samples were utilized; and this low baseline
makes the claimed difference in potency appear to be
greater than it has been in reality.
    In sum, the _new_marijaua_ is not new and neither is the
hyperbole surrounding this issue.  The implications of the
new disinformation campaign are serious.  Many people,
particularly the experienced users of the 1960's and their
children, will once again shrug off the warnings of drug
experts and not heed more reasonable admonishments
about more dangerous drugs.  This is not only abusive to
those who look to science, the medical profession, and
government for intelligent leadership, but will sully the
repuatations of drug educators who wittingly cry wolf, and
will inevitably diminish the credibility of drug abuse
treatment professionals who pass on such flawed reports.

(* end quote *)

[*] my only critisism of this article is that they included this
    reference to auto-titration.  Auto-titration is irrelevant in
    light of the fact that they show that the potency has not changed.
    Mentioning autotitration only serves to cloud the issue, since 
    there is some controversy over it.    - Lamont