I should clarify that the NDX "on average" during those years (since 1986) tops out within 8 days plus or minus 6 days.
That's equivalent to saying the NDX "on average" has a local top at least once a month. Which, of course, is a true statement for *every* month, not just January. By that logic there is never a reason to go long.
More data...
1993: NDX higher following the 8-day Jan window
1994: ditto
1995: ditto
1996: ditto
1997: ditto
1998: ditto
1999: ditto
2000: ditto
2001: ditto
2002: Ah, here's one, actually topped in early Jan this time
2003: And a second one...
2004: A wash...
2005: Nice decline on that one
From this its clear the odds of hitting new highs late in January is highly correlated to being (or not) in a bull market.
How...surprising!