Missouri Revised Statutes Title X. Taxation and Revenue § 138.050. Rules to be observed

(1) They shall raise the valuation of all tracts or parcels of land and all tangible personal property as in their opinion have been returned below their real value; but, after the board has raised the valuation of such property, it shall give notice of the fact, specifying the property and the amount raised, to the persons owning or controlling the same, by personal notice, or through the mail if address is known, or if address is unknown, by notice in one issue of any newspaper published within the county at least once a week, and that said board shall meet on the third Monday in July, to hear reasons, if any be given, why such increase should not be made; the board shall meet on the third Monday in July in each year to hear any person relating to any such increase in valuation. In any county with a charter form of government or any city not within a county, the board shall complete all business by the fourth Saturday in August. Any county of the first, second, third, or fourth classification shall complete all business by July thirty-first;

(2) They shall reduce the valuation of such tracts or parcels of land or any tangible personal property which, in their opinion, has been returned above its true value as compared with the average valuation of all the real and tangible personal property of the county.