A researcher wants to determine if a major economic recession in 2008 led to an increase in property crime. To do this, they obtain a dataset of all reported crimes from a large city's police department from 2005 to 2011. They plan to compare the property crime rates before and after 2008. However, they are concerned that the way the police department categorized 'property crime' may have changed during that period due to a new records management system implemented in 2009. Based on the constraints of using pre-existing data, what is the most significant challenge this inconsistency poses to the researcher's ability to draw a valid conclusion?