This study could have undercoverage bias because not all adults in the city may own or choose to download the fitness tracking app, which could lead to certain groups, especially those who are less tech-savvy or who may not be interested in tracking their fitness, being underrepresented in the sample.
Non-representative sampling might cause bias because the sample may primarily consist of younger, more active individuals who are likely to track their fitness, thereby skewing the results and not accurately representing the average daily step count of the entire adult population in the city.