Is this for the purposes of taking a position?
I can say that, over the last year, the FTSE100 has ended the week lower than the preceding week 31 times, whereas it has ended higher 20 times. So statistically, there is about a 60% chance the FTSE will end this week lower than 3830.1, last Friday's close. But this isn't a trading strategy or even the basis of one.
You could also see that if you had sold the close of each week, there were only 4 occasions when you could not have made any gain the following week, and on 2 of them the position would have come back into profit a week later anyway.
But none of this is trading.
Or am I wrong and people are actually making a profit this way?