Obviously the line is predicated on the starting pitching so it is the most important factor. However, in MLB there can be incredible value when looking at how a team has done the last 7, 10 or 14 days.
Remember the MLB EVERY team gets on a win streak and on a losing streak so look at the most recent data and don't be swayed by year-to-date numbers.
Best team LY in MLB (STL, 100-62) only won 61.7% of their games
Worst team in MLB LY (Philly, 63-99) won 38.9% - (MLB ONLY 22.8% from best to worst)
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Best team in the NFL (CAR 15-1) won 93.8%
Worst team in the NFL (TEN 3-13) won 18.8% - (NFL 75% from best to worst)
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Best team in the NBA (GSt 73-9) won 89.0%
Worst team in the NBA (PHI 10-72) won 12.2% - (NBA 76.8% from best to worst)
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So my advice is to find underdogs that have played better recently while their foes and been on a slide lately.