I’m assuming you’re looking at this factor with lower numbers being better. In general, anything that measures debt reduction would, in my opinion, be a good factor to include. Is this the best expression of debt reduction? I usually favor PYQ factors over PQ factors because of seasonality, but you may be able to make a good argument here. And perhaps using DbtTot2CapGr%PQ is better than DbtTotGr%PYQ. It hadn’t occurred to me before now. What matters is your reason for including it. Are you including it because you feel that your system needs to have some debt-reduction measure in it, and that this is really the best one because [I’m sure you can come up with a good argument for it]? In that case, great. Or maybe you have another debt-reduction measure in there and you think this would be a nice supplement to it. That would be fine too. In short, I can’t really think of many reasons not to include a debt-reduction factor that you think would work well with your system, especially if it gives your system a better backtested return. But, of course, you want to look at the alternatives and decide what debt-reduction factors make the most sense to you from a financial standpoint. Lastly, you’ll want to look at NAs here. If a company has no debt to reduce, it should ideally rank higher on this factor than a company that reduces its debt but still has a lot of it. So run a screen with this factor and look at the debt load of companies that score NA. If all of them are debt-free, perhaps you’ll want to use IsNA(DbtTot2CapGr%PYQ,-1000000) so that those companies have the lowest score.
One other thing: don’t use PQ factors on European stocks. Too many of them report semiannually, and PQ will be NA for those. PYQ and TTM should work fine.