Lately, Ive been noticing a large difference in attenuation between yeast vials, based upon their age.
My LHS sells a lot of yeast and for the more popular yeasts, I usually get vials with manufacturing dates less than 1 week old. For me, vials this fresh have been consistently outperforming vials that are over 3 weeks old. For example, in my last batch, the WL English (1 week old) had a 4% higher attenuation than the WL British (3.5 weeks old) and 2% higher for the WL Scottish (2 months old). Both are rated with higher attenuations than the English, the Scottish significantly so.
I do make large starters (2L per 5 gal. for ales , 4L for lagers). But the difference remains. Maybe Im not bringing the older yeast back up to prime condition with my starters (I dont use a stir plate, just oxygen injection). But, given my results, Im beginning to believe that there really is no substitute for very fresh yeast. The difference between 1 week old vials and 3 week old vials seems to be significant.