45. A study reported on 533,715 women at least 40 years old who were diagnosed with invasive breast cancer and reported to the National Cancer Data Base (NCDB). The study found strong evidence that patients without health insurance were more likely to have a more advanced stage of cancer (i.e., stage III or IV). Is this an experiment or observational study, and how do you know? [M. T. Halpern et al., Insurance status and stage of cancer at diagnosis among women with breast cancer, Cancer, 110(2) (2007): 403–411.]