I am curious about the effect of clumping on espresso. What we effectively do is create one big clump in the portafilter, yet clumping of some of the coffee that goes in to that big clump is said to degrade our result. Does anyone know of any studies that take object measurements like flow rate, channeling percentage, components of resulting coffee that show the effect of clumping? I hoping to avoid a discussion on whether clumping matters, I'm only interested in objective information that does not include subjective analysis like tasting.
Thx.

Have a look at this short post:

Simple Clumps and WDT Test: Espresso

My ZF64w produces some clumps. My Sette 270w does not. I don't do anything different in terms of puck prep between the two. I don't detect any consistent differences.

@Nunas - do you WDT?

No, I don't do anything much anymore, although I've tried nearly every gimmick. I grind the coffee straight into my portafilter, give it a few vertical thumps on my tamping stand, rock the tamper in a little circle (nutate), and lightly press (maybe only about five pounds or so), ensuring that the puck surface is dead flat. I have a group head manometer and can see if anything is going wrong, such as channeling; it rarely does. In recent years, the only gimmick that I've found to be effective is a sintered metal (IE the thick kind) puck screen. Although it does not seem to make any discernible difference in the cup, it most certainly keeps the group much cleaner.

I find that most of the "gimmicks" only apply to very lightly roasted coffees. What do you use in your gimmick-less routine?

I roast all my own coffee. I drop halfway between the end of 1c and start of 2c. So, mostly medium roasts.