This research explores the usability of the Federal Communication Commission's (FCC's) online Public Inspection Files to measure the sources and quantities of political advertising on broadcast television. We compared data from FCC files with data purchased from a commercial vendor in a presidential caucus campaign that stretched across nine months, including advertising sponsored by over 40 groups and totaled tens of millions of dollars. The FCC-derived and commercial data were consistent in reporting the quantity of advertising, but sponsor identification was inconsistent between data sources, raising concerns about the FCC's ability to disclose reliable information about political ad spending.