Some workers on MTurk are extremely active, and take the majority of posted HITs. This can lead to many issues, some of which are outlined in a blog post. Although MTurk has over 100,000 workers who take surveys each year, and around 25,000 who take surveys each month, you are much more likely to recruit highly active workers who take a majority of HITs. About 1,000 workers (1% of workers) take 21% of the HITs. About 10,000 workers (10% of workers) take 74% of all HITs.
TurkPrime now has a feature to allow researchers to exclude the most active workers so that you can collect data from less experienced workers who are less likely to have previously taken part in research similar to your own. Below is a screenshot of the “Naivete (Exclude most active Workers)” feature. You can select what percentage of workers you would like to exclude from the dropdown menu seen below.