TITLE: Is it highly unusual to win a lottery with 10 tickets in 6 weeks out of 7?
QUESTION [6 upvotes]: A woman in my local Rotary Club has won our weekly drawing 6 out of 7 weeks and I wanted to know the actual odds of this happening.  
On average 10 tickets are bought which I know makes her daily odds $1/10$. How to find the odds to win 6 out of 7?

REPLY [5 votes]: Most of the calculations assume that she was a person of interest BEFORE any of the draws, and there was a reason to be interested in those 7 draws. The question does not say that she was of special interest before, so those calculations are wrong. 
To give an example: if there are 10 tickets sold every week, and a person wins 2 weeks in a row, the probability is 1/100 for that person - unusual. However the probably of it happening to anyone in any given week is only 1/10. If this is a weekly draw happening for 2 years, it would be likely for this to have happened 10 times in the last 2 years, and 3 in a row should happen once. 
So to find the probability of your example happening randomly, you need to multiply the probability of a particular example happening by the number of ways it might happen. Every player is an extra chance, and every historical draw is an extra chance. You need to multiply the calculated probabilities by the number of people playing regularly (so times 10), but more significantly by the number of draws that there have been historically. Have there been 100 draws, 1000 draws? 
With 10 players and 100 draws, you need to multiply the probability calculated for her at this time by 1000. This makes the likelihood of this happening still unlikely, but not unlikely enough to be grounds for accusations of collusion.
If you then multiply this by the number of similar draws that happen in workplaces and clubs in just your city, it is bound to happen to someone sooner or later. 
Another method is to say that when she wins 3 in a row (should happen once every 100 draws), she becomes a person of interest, and you can calculate any further probabilities after that point. If her performance from this point remains highly unlikely (p<1%), maybe something is happening. 
As another answer noted, if there are multiple tickets bought by a single person, the odds can also quickly change, so make sure you take into account all the factors.