Rambam writes, based on a Gemara, that Yochanon Cohen HaGadol did a survey of how people were keeping hilchot trumah and ma-aser. Were there any other examples in the Gemara of chachamim actually going out, checking how people keep mitzvot, and actually changing halacha based on that survey? Did this ever happen in the post-Talmudic period? Ben