Public radio’s ratings have a disturbing habit of plunging over the weekend, mainly because commuters make up a large chunk of the daily listenership. And in fact, most weekend shows on public stations fit into a distinctive mold: one-hour blocks focused on specific niche audiences, with an eye towards cultivating a narrow but loyal listenership. Minnesota Public Radio is trying to change the model with a heavily researched new program designed around short, “bite-sized” news items and features, the better to fit into the average listener’s hectic weekend schedule. But an angry backlash against the show is building among industry insiders who are decrying the “Wal-Marting” of public radio.