What effect does stress have on cannabis-induced prospective memory loss?
A recently published study sought to examine the confounding effect stress might have on prospective memory loss seen in cannabis clinical trials. 
The science of statistics is not easy to understand, and I make no claim to be a statistician. But a cornerstone idea of statistical analysis is the differentiation between correlation and causation, and the relationship between them can be summed up as:
“Correlation does not imply causation.”
Imagine there are two variables, X and Y. Correlation occurs when changes in variable X align significantly with changes in variable Y, but the relationship between them is not directly connected. In other words, we can’t say for sure that changes in X will lead to changes in Y. Causation, on the other hand, occurs when changes in X are directly responsible for changes in Y. Causation produces the relationship between independent and dependent variables.
Scientific research looks for correlations and causations because two independent variables that are correlated can occasionally also be causal. But most times, their correlation is unrelated and caused by an unknown confounding (z) factor.  Consider the classic example: “When Ice Cream Sales Rise, So Do Homicides.” Or the data taken from U.S. government agencies shown on the graph below. 
“Prospective memory” is a term used to encompass the capacity of an intelligent network (such as an organism’s brain) for “forming a representation of a future action, temporarily storing that representation in memory, and retrieving it at a future time point.”  One of the most common primary adverse effects of cannabis or cannabinoid consumption in both clinical trials or recreational use is memory loss. [5-6]
Researchers at Washington State University examined the association between stress and prospective memory in cannabis vs. non-cannabis users.  The study established that acute stress, more so than cannabis use in particular, negatively impacts memory consolidation. However, this stress-induced negative impact can be exacerbated by cannabis use. Stress, therefore, may represent a hitherto unappreciated confounding factor in cannabis clinical trials.
Probing this interaction further is of particular importance to the medical field because conditions that qualify patients for medicinal cannabis use can induce a tremendous amount of stress, and potential memory loss represents an important factor that patients consider when deciding whether or not to incorporate medical cannabis into their medication routine. 
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- Crystal, J.D. and Wilson, A.G. “Prospective Memory: A Comparative Perspective.” Behav Processes, vol. 112, 2015, pp. 88-99 [Times cited = 13, Journal impact factor = 1.555].
- Volkow, N.D. et al. “Adverse Health Effects of Marijuana Use”. N Engl J Med, vol. 370, no. 23, 2014, pp. 2219–2227 [Times cited = 1367, Journal impact factor = 70.670].
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