Last calendar week , a study was published that claimed to found a link between casual marijuana use and abnormalities in the brains of recreational users . Intrigued by a title made by one of the paper ’s authors in the undulation of ensue press reporting , UC Berkeley computational life scientist Lior Pachter make up one’s mind to take a close look .
In reading the news last week I came across multiple reports claiming thateven casually smoking marijuana can change your brain . I usually do n’t pay much attending to such articles ; I ’ve never fume 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 distributor point of view , and as a citizen from a legal point of opinion , the issue have not been personal . However reading aUSA Today articleabout the report , I find that the primary investigator Hans Breiter was take to be a psychiatrist and mathematician . That is an unusual combining so I decided to take a closer look .
I now find out the call was a lie . In fact , the totality of math certification of Hans Breiter consist of somelogic / philosophy courses during a year abroadat 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 make one a doctor . Thus , with his outlandish claim , Hans Breiter had follow in personally offending me ! So , I decided to take a expression at his paper underlie the multiple newsworthiness reports :
J.M. Gilman et al . ,Cannabis Use Is Quantitatively Associated with Nucleus Accumbens and Amygdala Abnormalities in Young Adult Recreational Users , Journal of Neuroscience ( Neurobiology of Disease section ) , 34 ( 2014 ) , 5529–5538 .
This is quite possibly the high-risk paper I ’ve read all year ( as some of myprevious blog postsshow I am say something with this statement ) . Here is a crack-up of some of the military issue with the paper :
1. STUDY DESIGN
First of all , the survey has a very small sample size , with only 20 “ case ” ( marijuana users ) , a fact that is important to keep in judgment in what follows .
The title uses the term “ unpaid users ” to describe them , and in the insistence discharge follow the clause Breiter says that “ Some of these people only used marijuana to get high-pitched once or twice a week . hoi polloi think a little amateur use should n’t cause a problem , if someone is doing OK with workplace or school . Our data point directly state this is not the casing . ” In fact , the majority of users in the study were fume more than 10 joints per hebdomad . There is even a individual 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 “ refreshment ” then it seems to me that person is have a lot of fun ) .
More significantly , Breiter ’s instruction in the insistency release is a lie . There is no evidence in the paper whatsoever , not even a tiny shred , that the drug user who were get high once or doubly a week were having any trouble . There are also other publication with the study blueprint . For model , the theme claims the substance abuser are not “ maltreat ” other drug , but it is quite potential that they are getting gamy on cocaine , heroin , or ? ? ? as well , an issue that could quite perhaps affect the study . The experimentation lie in of an MRI scan of each user / control , but only a exclusive CAT scan was done . Given the variability in MRI scan this also seems baffling .
2. MULTIPLE TESTING
The study looked at three aspects of brain morphometry in the report participant : grey matter denseness , loudness and chassis . Each of these morphometric analyses constituted multiple trial run . In the typesetter’s case of grey matter concentration , estimate were based on lowly clump of voxels , result in 123 tests ( tie-up of each voxel cluster with marijuana use ) . volume were gauge for four regions : leave and right nucleus accumbens and amygdala . Shape was also quiz in the same four regions . What the authors should have done is to even out the atomic number 15 - values calculate for each of these tests by account for the total number of test performed . or else , ( Bonferroni ) corrections were performed separately for each case of psychoanalysis . For example , in the mass analytic thinking p - values were require to be less than 0.0125 = 0.05/4 .
In other Scripture , the extent of testing was not properly accounted for . Even so , many of the consequence were not meaning .
For example , the intensity analysis point no significant tie for any of the four test regions . The best case was the left nucleus accumbens ( Figure 1C ) with a corrected p - value of 0.015 which is over the author ’ own stated required doorsill of 0.0125 ( see caption ) . They use the language “ The connection with drug use , after correcting for 4 comparison , was compulsive to be atrend toward significance ” to describe this non - core .
It is worth noting that the removal of the outlier at a book of over 800 mm3 would almost certainly flatten the credit line altogether and remove even the little effect . It would have been nice to test this hypothesis but the authors did not release any of their data .
In the Fox News clause about the newspaper publisher , Breiter is quoted as saying the following :
“ For the NAC [ nucleus accumbens ] , all three measures were abnormal , and they were unnatural in a drug - dependent elbow room , meaning the change were cracking with the amount of marijuana used , ” Breiter enjoin . “ The corpus amygdaloideum had abnormalities for form and concentration , and only mass correlated with use . But if you bet at all three types of measurement , it usher the family relationship between them were quite abnormal in the marijuana exploiter , liken to the normal controls . ”
The result above show this to be a lie . Volume did not significantly correlate with use .
This is all very high-risk , but things get uglier the more one look at the newspaper publisher . In the tables reporting the p - value , the authors do something I have never seen before in a bring out paper . They report the uncorrected p - value , indicating those that are significant ( prior to correction ) in boldface , and then put an star 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 really insane . The fact that they put an star next to the economic value significant after correction designate they are aware that multiple examination is required . So why bother boldface phosphorus - values that they know are not meaning ? The overall issue is an impression that more test are important that is actually the case . See for yourself in their Table 4 :
The fact that there are multiple columns is also problematic . Separate run were performed for smoking occasion per day , articulation per social occasion , joints per hebdomad and smoking mean solar day per week . These measures are extremely correlated , but even so multiply test them requires multiple run rectification . The authors but did n’t do 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 word , they multiply the number of tests by four , and chose to not vex about that . Unbelievable .
Then there is Table 5 , where the generator did not report the p - values at all , only whether they were pregnant or not … without correction :
3. CORRELATION VS. CAUSATION
This government issue is one of the oldest in the Bible . There is evena wikipedia entry about it . Correlation does not imply causation . Yet despite the fact the every answer in the newspaper is directed at testing for association , in the last sentence of the nonfigurative they say “ These data suggest that marijuana exposure , even in immature amateur users , is associated with exposure - pendant alterations of the neuronic intercellular substance of core reward social structure and is ordered with animal written report of changes in dendritic arborization . ” At a minimum , such a result would require doing a longitudinal survey . Breiter take this language to an extreme point in the public press button accompanying the clause . I echo the affirmation 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 calendar week . mass think a little amateur use should n’t cause a trouble , if someone is doing fine with study or school . Our data directly says this is not the case . ” I consider that scientists should be approve for puddle public statements that directly contradict the content of their papers , as is likely the case here . There is case law for this .
This office byLior Pachteroriginally appeared at his web log , Bits of DNA , and has been republish with permission . His donnish site can be accessed here .
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