Studies have shown that hearing loss is related to the amount of noise pollution in cities. Suppose that researchers were interested in looking at the extent of hearing loss present among residents of Middelburg. To recruit participants they arrange a community event in which all residents can come to have a free hearing test. They obtain a very large sample of participants and measure their hearing as well as information on where they live and for how long. 

Among their conclusions are:

The participants had on average a higher level of hearing loss than the national average

The prevalence of hearing loss in Middelburg is higher than what is expected for cities of this size and level of development

1) Name the sampling strategy employed by the researchers.

2) Critically evaluate their conclusions in terms of external validity

3) Speculate on the types of people who would tend to be over or under-represented in the sample and describe the consequences this has for the external validity of this study.