Hi Bben. Thanks for bringing up useability. I’m highly skeptical of there being any meaningful correlation between the amount of cognition required to process a phrase, and whether that phrase uses an active or passive construction. I do however agree with the overarching sentiment of Content Design London’s guide. And this is precisely why we often need the passive — to make things shorter, clearer, and reduce the cognitive load.
Take my examples of “Martin kicked the ball” versus “the ball was kicked.” In the latter, there are only two concepts the reader needs to hold in their head: an object (the ball) and an action (kicking). Yet if we force this sentence into the active voice, we’re forced to add a second object — the agent of the action — even if that information isn’t relevant. This means there are now two objects and one action for the user to process, increasing the cognitive load, and muddying the mental image with an unnecessary element.
So sometimes (not always, but sometimes) using the passive will make for clearer, more concise copy.