The party believes that a good candidate, with a good campaign, could be able to obtain the Base GOP vote in a district plus two-thirds of the difference between the BASE GOP vote and the High GOP vote based on past elections. It is called optimal due to the two thirds formula. It is their most optimistic analysis. The lower the ORVS score the more Democratic the district is. A score of 48 an above means, in my estimation, that a Republican could have a chance in winning the election but no way is it a sure thing.
Strange things do happen.
Look at the Stockman seat. Stockman beat Jack Brooks. Also look at Bonilla's seat now held by a Republican but leaning for Democrats and Edward's seat now held by a Democrat but leaning Republican. Pete Geren's seat is a classic example. It leans Democrat and has been held by a Democrat but could go either way this Fall.