Well, I can say I was expecting more people to vote. Nevertheless, here is an example of do's and don't on wheels. All people who voted anything else except "filter abbreviated wheels" are correct; those who favor "filter abbreviated" are less correct as there is a better approach to try. I didn't include "open wheels" category on purpose as I expected people to to say that but they didn't.
I'd also be interested to learn what people use when they don't use wheels.
Anyway, here we go:
Whats going on with abbreviated open-cover/close-cover and full wheels
Suppose we have the following close cover C(9,6,5,6)=7 tickets [this means 9 numbers for a 6 game lottery gives 5 if 6 in 7 tickets]
1- 1 2 3 6 7 8
2- 1 2 4 7 8 9
3- 1 2 5 6 7 8
4- 1 3 4 5 6 9
5- 2 3 4 5 7 8
6- 2 3 5 7 8 9
7- 2 4 6 7 8 9
For ease of understanding, lets assume you have picked the numbers 1-9 to use and replaced 1 by 1, 2 by 2 etc. Close-cover wheel means 100% coverage. What that means for the above wheel? Well, 5 if 6 mean we can guarantee there is at least one ticket of the above 7 that gives a 5-hit, when all possible 6 ticket combinations (from 9 numbers in total) are checked=100% coverage (or we have to test C(9,6)=84 number combinations (this is the number displayed on top-right of covermaster).
Example: if the next draw is 1 2 3 5 7 9 (no ticket exists like this one in our wheel), we can see that ticket 6 above gives a 5-hit as promised. This is true if you check all possible 84 combinations: there will be at least one ticket that gives a 5-hit (maybe more but at least 1 will be there). So, we cover all 84 combinations and thus we have a coverage of 100%. In case the next draw is e.g. 1 2 3 6 7 30 (30 is outside our selected numbers thus 5 numbers matched), as you can see we still have a 5-hit (ticket 1) but in general we are not guaranteed this most of the time. CoverMaster offers a report tool to find out what is the coverage (and therefore chance) to win in that case too (the above wheel offers 30.95% to hit 5if5).
Now, lets have a look at an C(9,6,5,6,80%)=5 (that is 9 numbers 5if6 80% coverage needs 5 tickets). Well, if you check this wheel in covermaster, it actually offers 89.28% coverage (or 75 out of 84 possible C(9,6) combinations). So, we better say this is an C(9,6,5,6,89.28%)=5 open-cover wheel.
1- 2 3 4 5 6 7
2- 1 3 6 7 8 9
3- 2 3 4 5 8 9
4- 1 2 4 5 7 9
5- 1 2 4 5 6 8
The idea is exactly the same as in close-cover wheel. But now we can cover only 75 of the total 84 combinations and this is why we have 89.28% coverage. For example, if the next draw will be 4 5 6 7 8 9, there is no ticket in the 5 above that can give you a 5-hit. This 4 5 6 7 8 9 ticket is one of the 84-75=9 not covered.
To get an idea why it is bad to remove (or filter lines) from an abbreviated wheel (either open or close), Ill remove 2 lines from the first close cover wheel (Ill remove the last 2).
1- 1 2 3 6 7 8
2- 1 2 4 7 8 9
3- 1 2 5 6 7 8
4- 1 3 4 5 6 9
5- 2 3 4 5 7 8
The coverage of this wheel is 72/84=85.71% (worse than 75 of our open-cover wheel above even if they have the same length. This is the case, whichever lines you remove from the close-cover wheel). Still, the above is an open-cover wheel but not that good one as you understand. Now, imagine how much worse things go in terms of coverage with larger wheels. This is why no filtering suggested to such wheels (both open & close abbreviated wheels).
Now, lets see whats happening on full wheels and their coverage. An C(9,6,6,6)=84 wheel offers 100% coverage which means whichever of the C(9,6)=84 combinations come out, there is only and only one ticket that covers it. This is obvious as each ticket of the full wheel is one of the 84 possible combinations. Thus each ticket of our full wheel corresponds and covers only one of the 84 possible combinations. Therefore, if you remove a ticket from the full wheel, you simply do not cover that particular combination. The rest remains unaffected. This is why you can filter full wheels because when filtering, what you suggest is what the winning ticket will NOT look like.
The conclusion of the above is that we can filter full wheels but not abbreviated open/close cover wheels. In that case, it is prefferable to use a smaller wheel size of the desired length (e.g. an open-cover wheel).