Observational equivalence

One of the measures of PSB independence I'm looking at is the turn-over rate of PSB directors-general, and specifically whether they often change within six months of a government change. The idea is that, if directors-general are appointed only for two years, or if they know they'll be fired with a government change, they can't be independent of politics.

So, if turnover is low,that means the broadcaster is independent, doesn't it? Not necessarily. Here's Mark Pollack explaining why the problem of observational equivalence messes things up:

Studies of agency autonomy that rely on the frequency of sanctioning are therefore likely to run into the methodological problem of observational equivalence, namely that the absence of sanctions is consistent with both the obedient servant and the runaway bureaucracy scenarios, each of which predicts, albeit for different reasons, the rarity of sanctions