Question: On a talkshow this morning during a discussion of the validity of random drug tests a member of the Illinois State Police stated that 70% of admitted Marijuana users also admit to using while at work. I am not finding any studies or statistics that back this up or refute it. 

The claim was used to support the requirement that teachers be subject to random drug testing. That claims of what is done at home stays home is most often not true. I had not originally included this part as my skepticism is about the usage at work not about the effectiveness of drug testing.

Answer: 
While this doesn't *prove* that the hypothesis "70% of marijuana users also use while working" is false, it certainly makes it look unlikely.

<http://www.icpsr.umich.edu/quicktables/quicksetoptions.do?reportKey=23782-0001_du%3A7>

Using data from this site, I was able to come up with the following table. This represents employed (both full-time and part time) people who say they have *ever* used marijuana, and how frequently they have used over the last 12 months:

```
77.2% have not used at all in the last 12 months
6.5% have used 1-5 days in the last 12 months
1.5% have used 6-10 days in the last 12 months
2.7% have used 11-25 days in the last 12 months
3.6% have used 26-100 days in the last 12 months
8.4% have used 100+ days in the last 12 months

```

If we only consider the "recent" users (excluding the 77.2% who have not used in the last 12 months), we are left with:

```
28.5% have used 1-5 days in the last 12 months
6.7% have used 6-10 days in the last 12 months
12.0% have used 11-25 days in the last 12 months
16.0% have used 26-100 days in the last 12 months
36.8% have used 100+ days in the last 12 months

```

If this data is accurate, for 70% of current marijuana users to smoke at work, then even a good portion of even the most casual users would have to smoke at work, and that just seems unlikely. I suspect the most casual users are the least likely to smoke at work.

So I would argue that for any *useful* meaning of statistics, it is clear that 70% of pot users do not use while at work.

Of course, by massaging the numbers sufficiently, it may be possible to claim that 70% of people who have used at any time in their life, used at work at some time in their life--or more likely were high at work (used immediately before arriving to work, or over their lunch break, etc). But using such a statistic for anything other than curiosity, I would consider unethical, considering that 77% of "some time in their life" users no longer use at all, according to this data.
For the total user population, 70% cannot have used marijuana in the workplace.
I derived from [table 2.11 on this paper](http://oas.samhsa.gov/NSDUH/2k9NSDUH/2k9Results.htm#2.13) to find that 33.6% of marijuana users aren't even employed. So if 33.6% aren't employed it can't be true that 70% are using at work.

33.6% is reached by using the 2009 data on millions of users (*NOT* the rate of use in past month chart) graph here. Unemployed + Other + Full Time + Part Time/ Unemployed + Other. The other column is people who are not in the workforce, but not unemployed. 

(9.2+3.6+2.5+4.0)/(2.5+4.0) = .336

![Table of users](https://i.stack.imgur.com/xzIqf.gif)

However, the actual claim in the show could still have been true. The likelyhood is that the officer was just quoting a study. Since the [vast](http://oas.samhsa.gov/Highlights2k10.htm) [majority](http://www.nida.nih.gov) of studies use "admission" to mean admittance to a medical facility (which is the standard definition used in medical research) and *not* admitting to use, then 70% could easily be an accurate number. People who are admitted to a medical facility for abuse are going to be centered towards the most severe and frequent users and would not represent the general population. Indeed, [almost 30% of admitted users were referred *by their employer*](http://oas.samhsa.gov/2k10/304/304Marijuana2k10.htm), so the number of admitted users that use at work could realistically be within 70% (no this doesn't show it, but it does show that 70% isn't a wild crazy made up number).