Editing Natural experiment (section)

===Smoking ban===
In [[Helena, Montana]] a [[smoking ban]] was in effect in all public spaces, including bars and restaurants, during the six-month period from June 2002 to December 2002. Helena is geographically isolated and served by only one hospital. The investigators observed that the rate of [[myocardial infarction|heart attacks]] dropped by 40% while the smoking ban was in effect. Opponents of the law prevailed in getting the enforcement of the law suspended after six months, after which the rate of heart attacks went back up.<ref name="Sargent_1">{{cite journal |last1=Sargent |first1=R. P. |last2=Shepard |first2=R. M. |last3=Glantz |first3=S. A. |title=Reduced incidence of admissions for myocardial infarction associated with public smoking ban: before and after study |journal=[[British Medical Journal]] |volume=328 |issue=7446 |pages=977–980 |year=2004 |doi=10.1136/bmj.38055.715683.55 |pmid=15066887 |pmc=404491}}</ref> This study was an example of a natural experiment, called a [[crossover study|case-crossover experiment]], where the exposure is removed for a time and then returned. The study also noted its own weaknesses which potentially suggest that the inability to control variables in natural experiments can impede investigators from drawing firm conclusions.'<ref name="Sargent_1" />