Skashion said:
Best defence is judged by goals conceded and cleans sheets, not tackles and shots conceded because one shot is not equal to another, ditto tackles.
Neither are perfect indicators in isolation, but football, like anything, is ultimately a game of probability. A team could concede lots of shots, but face teams continually lacking their shooting boots, lucky deflections, etc. 0 goals conceded. Alternatively a team could concede almost no shots, but face teams who are unerringly clinical, and knock home the 3 chances they get all game. Both scenarios will play out plenty of times this season.
The fewer shots you concede, the lower your chances of letting in goals. This is undoubtedly a sign of a better defence. I imagine if you were to take a large enough sample size the best defences in the league would most often concede both fewest shots AND concede the fewest goals (I don't think clean sheets is a particularly good measure, but again, it would probably correlate closely).
As it stands we simply don't have enough information on Pellegrini's City side to properly determine whether the defence has improved or not. A simple change in formation could be enough to skew the results hugely this early on. Will be interesting to see the numbers at the end.