We use the synthetic control method to analyze the effect of face masks on the spread of Covid-19 in Germany. Our identification approach exploits regional variation in the point in time when face masks became compulsory. Depending on the region we analyze, we find that face masks reduced the cumulative number of registered Covid-19 cases between 2.3% and 13% over a period of 10 days after they became compulsory. Assessing the credibility of the various estimates, we conclude that face masks reduce the daily growth rate of reported infections by around 40%.(Institute of Labor Economics, June 2020)

(Note:A "synthetic control method" refers to the definition of a control group as a composite of groups, instead of just one group. In this case, groups = regions of Germany.)

Which of the following is true about the authors' conclusion

Question 5 options:

a)

The authors can conclude causality because this is a natural experiment.

b)

The authors can conclude causality because this is a field experiment.

c)

The authors cannot conclude causality because this is an observational, as opposed to experimental or quasi-experimental study.

d)

None of the above.