PPDA cannot compare two different leagues

The metric divides by what the opponent chose to do, which makes it a measure of two teams dressed up as a measure of one.

Passes allowed per defensive action is a neat idea. Count the opponent's passes in their own build-up area, divide by your tackles, interceptions, fouls and challenges, and you get a number where low means aggressive. It works well enough inside one league across one season. The moment it travels, it stops meaning anything, and it gets used to travel constantly.

The problem is the denominator's dependence on somebody else's behaviour. If the opponent chooses to knock it long from goal kicks, your PPDA improves without you doing anything at all, because the passes you were meant to allow never happened. If the opponent circulates the ball patiently across his back four, your PPDA worsens even if your press is identical, because he is generating passes you cannot reach.

Now put that in two competitions with different habits. A league where most sides go long produces flattering PPDA figures for everybody. A league of possession teams produces ugly ones. Then a recruitment department compares a pressing side from the first against one from the second and concludes something confident and wrong. I have seen this argument made in public more than once and the correction never travels as far as the original claim.

There is also a definitional mess underneath. What counts as a defensive action varies by data provider, and fouls in particular are treated inconsistently. A cynical tactical foul is an intervention, so it lowers PPDA, so a side that presses badly but hacks people down looks like a side that presses well. That is not a rounding error. For some teams it moves the figure noticeably.

The fix is not complicated but it is unglamorous. Adjust for the opponent's build-up style, report it as a distribution across matches rather than a season average, and stop quoting it as a single figure. Better still, look at where the ball is when the action happens, because a press that wins the ball level with the opposition penalty area and one that wins it on halfway are different tools with the same score.

Use it as a rough sorting device inside one competition. Stop treating a ratio built from the other team's decisions as a description of yours.