Parlaying O/U and ATS

Every sporting event with a spread and an over/under has four possible combinations of outcomes, disregarding a push:

• FAV + OVER
• FAV + UNDER
• DOG + UNDER
• DOG + OVER

Most books let you parlay these combinations together on the same event, for the standard 2-team payout of 13/5 (+260). So is there any value to doing so? In theory, each combination should occur ~25% of the time. At +260, you need to hit above 27.78% to turn a profit (parlays are very rarely an efficient bet compared to individual ones).

I looked at the last 5 seasons for NBA, NFL, and NCAAF (NCAAB data can be spotty, especially for O/U) to see if any trends stood out. Over that many games nothing hurdles above 27.78%:

NBA	Actual	%	Exp	Dif
FAV + OVER	1,515	23.94%	1,582	-66.75
FAV + UNDER	1,638	25.89%	1,582	56.25
DOG + UNDER	1,561	24.67%	1,582	-20.75
DOG + OVER	1,613	25.49%	1,582	31.25
Total	6,327	100.00%
NFL	Actual	%	Exp	Dif
FAV + OVER	282	24.02%	294	-11.5
FAV + UNDER	289	24.62%	294	-4.5
DOG + UNDER	304	25.89%	294	10.5
DOG + OVER	299	25.47%	294	5.5
Total	1,174	100.00%
NCAAF	Actual	%	Exp	Dif
FAV + OVER	917	25.80%	889	28.5
FAV + UNDER	857	24.11%	889	-31.5
DOG + UNDER	920	25.89%	889	31.5
DOG + OVER	860	24.20%	889	-28.5
Total	3,554	100.00%

What about at the extremes, when one team is favored by a lot? Some small trends did stand out on larger spreads, albeit at smaller sample sizes of course.

NBA really doesn't do much. 7% of games matched the criteria of a spread >= 12, and there's not much of a return to be had:

NBA, ATS >= 12	Actual	%	Return	ROI
FAV + OVER	80	18.02%	-156	-35.14%
FAV + UNDER	124	27.93%	2	0.54%
DOG + UNDER	129	29.05%	20	4.59%
DOG + OVER	111	25.00%	-44	-10.00%
Total	444	100.00%

NFL does it a bit better. 12% of games had spreads >= 10, and there's some nice value in combining the underdog and the under:

NFL, ATS >= 10	Actual	%	Return	ROI
FAV + OVER	41	28.08%	2	1.10%
FAV + UNDER	30	20.55%	-38	-26.03%
DOG + UNDER	45	30.82%	16	10.96%
DOG + OVER	30	20.55%	-38	-26.03%
Total	146	100.00%

In the end, NCAAF has the largest opportunity, with 18% of games having spreads >= 20.5 and some really nice value pairing the favorite and the over:

NCAAF, ATS >= 20.5	Actual	%	Return	ROI
FAV + OVER	204	31.83%	93	14.57%
FAV + UNDER	129	20.12%	-177	-27.55%
DOG + UNDER	164	25.59%	-51	-7.89%
DOG + OVER	144	22.46%	-123	-19.13%
Total	641	100.00%