Choosing an arbitrary cutoff of 6 years and then assuming that everyone who didn't complete their degree within 6 years is a college dropout is not a particularly valid way to measure dropout rates. I completed my degree after the 6-year mark and I'm certainly not the only one to do so. These statistics erroneously include people like me in the dropout category. A more accurate method of measurement would be to track each cohort of students from entrance through graduation and chart a histogram of the elapsed time from initial enrollment to graduation. Once you've collected these histograms for several cohorts, you're then in a position to construct a model of the statistical distribution. Then you can make accurate estimates of the true dropout rate by fitting the known graduation data for a given cohort to the model to estimate how many of the students are in the tail of the distribution that will eventually graduate.