By Lior Pachter
In reading the news I
came across multiple reports claiming that even casually
smoking marijuana can change your brain. I usually don’t pay much attention to such articles; I’ve never smoked a joint in
my life. In fact, I’ve never even smoked a cigarette.
So even though as
a scientist I’ve been interested in cannabis from the molecular biology point
of view, and as a citizen from a legal point of view, the issues have not been
personal. However reading a USA Today article about the paper, I noticed that the principal
investigator Hans Breiter was claiming to be a psychiatrist and mathematician.
That is an unusual
combination so I decided to take a closer look. I immediately found out the
claim was a lie. In fact, the totality of math credentials of Hans Breiter
consist of some logic/philosophy courses during a year abroad at St. Andrews while he was a pre-med student at
Northwestern. Even being an undergraduate major in mathematics does not make
one a mathematician, just as being an undergraduate major in biology
does not makes one a doctor.
Thus, with his
outlandish claim, Hans Breiter had succeeded in personally offending
me! So, I decided to take a look at his paper underlying the multiple
news reports:
This is quite
possibly the worst paper I’ve read all year (as some of my previous blog posts show I am saying something with this statement).
Here is a breakdown of some of the issues with the paper:
1. STUDY DESIGN
First of all, the study
has a very small sample size, with only 20 “cases” (marijuana users), a
fact that is important to keep in mind in what follows. The title uses the term
“recreational users” to describe them, and in the press release
accompanying the article Breiter says that “Some of these people only
used marijuana to get high once or twice a week. People think a little
recreational use shouldn’t cause a problem, if someone is doing OK with work or
school. Our data directly says this is not the case.” In fact, the majority of users
in the study were smoking more than 10 joints per week. There
is even a person in the study smoking more than 30 joints per week (as
disclosed above, I’m not an expert on this stuff but if 30 joints per week is
“recreation” then it seems to me that person is having a lot of fun). More
importantly,
Breiter’s
statement in the press release is a lie.
There is no evidence in the paper whatsoever, not even a tiny shred, that the
users who were getting high once or twice a week were having any problems.
There are also other issues with the study design. For example,
the paper claims the users are not “abusing” other drugs, but it is quite
possible that they are getting high on cocaine, heroin, or ??? as well, an
issue that could quite possibly affect the study. The experiment consisted of
an MRI scan of each user/control, but only a single scan was done. Given the
variability in MRI scans this also seems problematic.
2. MULTIPLE TESTING
The study looked at
three aspects of brain morphometry in the study participants: gray matter
density, volume and shape. Each of these morphometric analyses constituted
multiple tests. In the case of gray matter density, estimates were based on
small clusters of voxels, resulting in 123 tests (association of each voxel
cluster with marijuana use). Volumes were estimated for four regions: left and
right nucleus accumbens and amygdala. Shape was also tested in the same four
regions.
What the authors should
have done is to correct the p-values
computed for each of these tests by accounting for the total number
of tests performed. Instead, (Bonferroni) corrections were performed separately
for each type of analysis. For example, in the volume analysis p-values
were required to be less than 0.0125 = 0.05/4.
In other words, the extent of testing was not
properly accounted for. Even so, many of the results were not significant. For example, the volume
analysis showed no significant association for any of the four tested regions.
The best case was the left nucleus accumbens (Figure 1C) with a corrected p-value
of 0.015 which is over the authors’ own stated required threshold of 0.0125
(see caption).
They use the language
“The association with drug use, after correcting for 4 comparisons, was
determined to be a trend toward significance” to describe this non-effect. It is worth noting that
the removal of the outlier at a volume of over would almost certainly flatten the
line altogether and remove even the slight effect. It would have been nice to
test this hypothesis but the
authors did not release any of their data.
 |
| Figure 1c |
In the Fox News article
about the paper, Breiter is quoted saying ““For
the NAC [nucleus accumbens], all three measures were abnormal, and they were
abnormal in a dose-dependent way, meaning the changes were greater with the
amount of marijuana used,” Breiter said. “The amygdala had abnormalities
for shape and density, and only volume correlated with use. But if you
looked at all three types of measures, it showed the relationships between them
were quite abnormal in the marijuana users, compared to the normal controls.”
The result above shows this to be a lie. Volume did not significantly correlate
with use.
This is all very bad,
but things get uglier the more one looks at the paper. In the tables reporting
the p-values, the authors do something I have never seen before
in a published paper. They
report the uncorrected p-values,
indicating those that are significant (prior to correction) in boldface, and
then put an asterisk next to those that are significant after their
(incomplete) correction.
I realize my own use of boldface is
controversial… but what they are doing is truly insane. The fact that they put
an asterisk next to the values significant after correction indicates they
are aware that multiple testing is required. So why bother boldfacing p-values that they know are not
significant? The overall effect is an impression that more tests are
significant that is actually the case. See for yourself in their Table 4:
 |
| Table 4 |
The fact that
there are multiple columns is also problematic. Separate tests were performed
for smoking occasions per day, joints per occasion, joints per week and smoking
days per week. These measures are highly correlated, but even so multiply
testing them requires multiple test correction.
The authors simply didn’t
perform it. They say “We did not correct for the number of drug use measures
because these measures tend not be independent of each other”. In other words,
they multiplied the number of tests by four, and chose to not worry about that.
Unbelievable.
Then there is Table 5,
where the authors did not report the p-values at all, only
whether they were significant or not… without
correction:
 |
| Table 5 |
3. CORRELATION VS. CAUSATION
This issue is one of
the oldest in the book. There is even a wikipedia entry about it. Correlation does not imply causation. Yet
despite the fact the every result in the paper is directed at testing for
association, in the last sentence of the abstract they say “These data suggest
that marijuana exposure, even in young recreational users, is associated with
exposure-dependent alterations of the neural matrix of core reward structures
and is consistent with animal studies of changes in dendritic arborization.” At
a minimum, such a result would require doing a longitudinal study. Breiter
takes this language to an extreme in the press release accompanying the
article.
I repeat the statement
he made that I quoted above where I boldface the causal claim: “”Some of these
people only used marijuana to get high once or twice a week. People think a little recreational use
shouldn’t cause a problem, if someone is doing OK with work or school. Our data directly says this is
not the case.” I believe that scientists should be sanctioned for
making public statements that directly contradict the content of their papers,
as appears to be the case here. There is precedent for this.
